On Sunday 2nd April 1911, the decennial UK Census recorded the Munros at their home in a flat at 23 Douglas Place (now renumbered as 3, 5, 7 and 9 Douglas Place):

Here is the Census record:

Parents Robert and Mary, and six of their children, lived in three rooms; the absentee was their second son Robert (there were five boys and two girls). Four of the children had left school and had jobs.
The father, Robert, was 48 and from Springburn. He was a coachman in a posting yard (carriages for hire). He could have worked for Mackie and Denholme, in the recently built premises on Kirk Road called Kirk Stables (link).
Mother Mary Brown was 43 and from Hardgate. She gave birth to nine children, seven of whom survived to 1911.
Their eldest daughter Maggie, 23, was assistant dressmaker in a dressmaking warehouse
Their eldest son David, 20, was an assistant baker, working for Gilbert Hamilton in his bakery and bakehouse at 2 Stewart Place (on the corner of New Kirk Road and Drymen Road, currently Okome)
Their third son Thomas,16, a ‘boy’ (a dogsbody, I assume) in a clothing warehouse
William, 14, had left school and was a messenger boy
John, 12, and Mary, 10, were still at school, very probably Bearsden Public School just across Roman Road.
1913 was a momentous year for the Munro family. In January, Margaret (Maggie) married Arthur Allison, an engineer and son of the farmer at Thorn Farm. On 27th December Thomas was married to Susan Kerr, who worked in a fruiterer’s shop in Townhead; they made their home at Weir Street in Tradeston (now gone, probably renamed Houston Place – opposite side of Paisley Road to Springfield Quay). The following afternoon, their father Robert died at 23 Douglas Place of pneumonia.
Following the declaration of war, three of the four sons who were eligible (Tom, Robert and David) had joined the army and their photos were included in the 1916 book “The Call to Arms: New Kilpatrick’s Response”, published in that year:

Thomas (Tom) was an acting corporal with 5th Cameronians in May 1917 in northern France between Arras and Bapaume, near Croiselle. On the 15th they were given notice that in the next few days they would attack a feature in the German line called The Hump. Reconnaissance proved difficult because the Germans had chosen the position so that it could not be overlooked. However, a model had been built in the fields behind the British line so that the Cameronians could practice their assault.
This map gives some idea – the German front line is in red, the British line in dark blue. The Hump was approximately where two red trenches join just to the left of centre:

(credit: The Long Long Trail website (link)
The battalion moved up to attack on the night of the 19th to 20th, but fog and smoke from shell bursts made it difficult to spot locations. Two companies made it into the German trenches but the other company met more opposition and dug in in front of the German barbed wire. The Germans attacked steadily, including from the flanks and no further progress could be made. (I’m indebted to Tom Lang of The Great War Forum (link) for having transcribed the diary and made it available.)
Somewhere in all this, Thomas Munro was killed. He may have been buried later on, but if so the location of his grave was lost. He is listed on the Arras Memorial:

The Memorial is the building in the background, its walls covered with lists of the names of almost 35,000 troops from Britain and allied countries who died in the area and have no known graves.
Here is the record of what happened to Tom’s pension:

As the card notes, Susan remarried Walter Muir on 28th September 1917.
News that Tom was dead would have reached Scotland a few days later. We can only speculate on what effect this had on his brother, William. He had appealed against his call-up just under a year earlier (Milngavie and Bearsden Herald 9th June 1916):

William was having a busy time – three weeks later (22nd June) he was married to Annie Pender, a domestic servant, who lived in St Georges Road. Evidently she was heavily pregnant because their son, Robert, was born two weeks later (4th July 1916) at 33 Murray Street, just off Parliamentary Road in Townhead. They subsequently lived at Garscadden Rows (the Fourth Row).
It’s unclear how, but William was granted exemption from the call-up as he was a vanman (driver) for Gabriel Hamilton who stated he had employed 11 men before the war and now had three.
This exemption was specific to his job, but in June 1917 (a few weeks after the news of Tom’s death) he was sacked. His employer was required to inform the local recruiting office in Dumbarton:

The recruiting office received a second letter from the clerk to the local Parish Council:

The recruiting office wrote to him a week later, giving a time and date to report but he did not do so; visited by the local policeman on 2nd August he claimed not to have received the letter. Finally in the army on 29th August 1917, he was sent to Dreghorn Barracks in Edinburgh where he was found to be 5 feet, 5 inches, 146 pounds and to have “good physical development” but “defective teeth” for which he had dentures made.
On 25th September Annie gave birth to their second child, Janet, and six days later, William went absent without leave from his barracks in Edinburgh:

I cannot see any evidence of what he did in the next three weeks before he was “apprehended at Glasgow on 23rd October 1917 by the Civil Power”, being collected from the police station at Queens Park. I hope he went to see Annie, Robert and Janet.
Possibly softened to some extent by the circumstances around his disappearance he appears to ‘only’ have been fined from his pay (the days he was absent plus an additional eight days), when he could have faced a far harsher penalty.
On 5th March 1918 he was sent to France and went into the front line as a private in the 10th Argyll and Sutherland Highlanders (97th Brigade, 32nd Division). Several weeks later, the battalion was in the front line at Houtholst Forest, north-west of Ypres and on the edge of the infamous Passchendaele battlefield. An idea of the environment:

(photo credit: Eddy Lambrecht, thanks also to Cnock for positing it in The Great War Forum (link)).
The battalion’s war diary for 25th March reads: “11pm Orders received to move at short notice – Hostile shelling on lamp (lump?) at ABRI [probably the name of a trench] – a few casualties.”

William was probably one of the casualties. He is recorded as having died on the 26th and the diary entry for the following day shows the battalion marching back to be moved by train. He was buried at Artillery Wood Cemetery:

(Photo credit: tracesofwar.com (link))
Back at Garscadden Rows, baby Janet died on 2nd November 1918, of influenza and broncho-pneumonia so possibly a victim of the so-called Spanish Flu epidemic. William’s widow, Annie, married a coal miner, James George and they moved to 37 Albert Street, Harthill. Annie developed TB and died in the sanitorium in Shotts on 12th February 1921, aged 25. The 1921 Census shows her husband James had moved back in with his family in Shotts, with 5-year old Robert living next door with relations at Hassockrigg Row.

This leaves Robert. He was a private with the 7th Argyll and Sutherland Highlanders, who were part of the 51st Highland Division. In March 1918 this battalion was in the front line near the north-east edge of the 1916 Somme battlefield. On the 21st of that month, the Germans launched attacks all along the line held by Britain and her allies using troops freed up by the withdrawal of Russia from the war in 1917. The Germans broke into the battalions trenches and started working along them using grenades and flamethrowers. It was the start of five days of non-stop fighting, followed by successive retreats, with little food, no sleep and cold, coming out of the front line on the 26th with 326 men killed, wounded or missing (out of perhaps 700 at the start).
I have copied a small section of the war diary below to give you a flavour:

One of the wounded was Robert, and he died of his wounds on the 30th March. It’s highly likely he had been evacuated to a hospital well behind the front line by this time because he is buried at Etaples Military Cemetery. This was the burial ground for several local army hospitals.

(photo credit CWGC)
In a report of the fighting from 21st to 26th, the colonel commanding Robert’s battalion wrote:

It seems from the final comment that Robert was one of the few who was evacuated in this way.
The official record shows the allocation of his effects, presumably his back pay:

In the right hand column you can see allocations to his mother, his surviving siblings (David passes his share to his mother, as well as the wives of Tom and William.