Ok. Firstly: Going by Wikipedia again
D), the kids would probably have been evacuated around 1939/40, and ought not to have returned until 1945. Though like Animus says - that may just be officialy. However, London did seem to be stricter than the other towns so I think we should say they didn't return till 1945.
If we say that LWW happened in early summer 1940, then PC would be late summer 1941. Now as Animus quoted, VDT must have happened after the war in the summer. By the way it says 'long ago in the war years' and that 'mother hadn't had a proper holiday for ten years' we can definitely say it's no later than 1948 (mum couldnt have gone on holiday during the war), but probably not much earlier otherwise 'long ago' wouldn't make sense. Now we actually dont know how long after PC VDT takes place. We only know it's more than a year - 'Last year, when he had been staying with the Pevensies, he had managed to hear them all talking of Narnia'. This also suggests that last year was post-war as the Pevensies were at home. I think 1946-8 is a suitable time frame for VDT.
SC happens in the autumn following so that would also be 1946-8 and in LB Eustace says they were last in Narnia 'more than a year ago by our time'. That puts LB at some point in 1948-50 - I would guess summer.
Now, I spoke to my gran about schools around the time. Bear in mind she went to school from 1925-1937 so it's a little before but here's what she said:
Infants: 4-7
Elementary school: 7-11/14
At 11 children would take an exam to get into grammar schools and if they passed they would go. Those who didnt stayed then left school at 14.
Grammar school: 11-16
My Gran says she took O levels. She said most people left at 16. Not many went further. She tried herself but left after a term or two.
Now she didn't know about boarding schools which to me suggests that those richer children would have gone through a different education system. I had a look online and it seems that children can start boarding at any age which unfortunately means we cant pinpoint Lucy...
I think one place we can start is with Peter. I looked more at the mention of Peter's exam and it struck me that it is just one exam! This to me makes me think it's a university entrance exam rather than school exams. Also if he's studying over the holidays - after school is finished - it would make more sense. And furthermore, he's gone to study with a Professor - it must be a high level exam for that so I say Peter is 18 in VDT, preparing for uni.
Now if Peter is 18 in VDT which is somewhere between 1946 and 48 then in LWW he would be between 10 and 12. 12 seems most likely as any younger would be too young. This means VDT and SC are in 1946 and LB is in 1948.
We know that Eustace and Jill are not at boarding school because it is mixed, yet I think Experiment House would be a Grammar School because Eustace seems rather intelligent. Considering both were at school the year before SC takes place then they must be at least 12 in SC. This would put their age around the events of LWW as being 6.
Therefore, the ages of Lucy, Edmund and Susan have to range between 7 and 11. Susan could certainly be 11, though I would put Edmund as 10 and Lucy as 8.
If we move to LB, we read that Peter is described as not quite 'full grown' but with a face like a king or warrior, and 'you could almost say the same of' Edmund. Lucy is described as a girl and Jill and Eustace as 'a boy and girl younger still'. We also know Edmund had an injury from a game of rugby. You'd think he'd still be at school if he was playing rugby so couldn't be older than 18. If we go by the above calcultions their ages in LB would be:
Peter: 20
(Susan: 19)
Edmund: 18
Lucy: 16
Jill/Eustace: 14
I think those ages fit those descriptions, although Lucy could perhaps be even a little bit younger maybe 15. Otherwise she's not really a girl.
Therefore, my final answer to the thread question is:
In LWW the Pevensie children are aged: Peter 12, Susan 11, Edmund 10, Lucy 7/8
Though in all honesty, Mr Bob is probably most accurate. I don't think Lewis really thought it through.