How tall you are would seem to be a large factor of how much bending you will need to do and how sore your back might get.... 
5', 6'6", other???
Personally I go for 42" now, and I'm right about 6' tall. (though I do not have kids I was trying to build it for - and I am only approaching middle age (40s), so not too worried about bending/reaching yet). My dad used to go with 36", which was fine as I was growing up, but I was older by the time I was spending significant time on my own layout (HO at the time, but shouldn't affect the goal much).
-Dave
