(Bear with me, I'm off ill and got too much time to think.)
I was just thinking about the Secret Garden - it's been made into another film; out this year.
It's set in 1910 (or 11) Dickon was 12 I think (?). So will be 16 in 1914. Good chance he's off to the trenches at some point, highly possible he dies there.
Colin was 10 so will miss out on being having to go and probably continued being obnoxious and privileged. (Might be hard to tell, but I didn't like Colin.) He would probably miss being conscripted - as he would be 39 when WW2 started and probably married. (Plus I can't recall when the upper-aged men got conscripted, it wasn't immediately and i think Colin would probably have just missed out age-wise).
Mary I think would be married off and that would be that for her. If she's lucky she might have a passionate affair with the gardener.(Probably not Ben Weatherstaff; thinking more along the lines of Oliver Mellors.)
Martha (my favourite character) I think would have married, had many children and lived near her mother.
I'm going to figure out Enid Blyton characters now. (Despite a lot of them being knob heads.)