Late to the party which is usual for me but interestingly enough my D40 still has an important feature that makes me bring it along on daytime shoots. Unlike any of my other cameras, it can flash synch at 1/500s which makes it the bomb for balance fill flash photography. As a teaching instrument it is much more friendly than my D70 which I assume is similar to a D50. My D300 and D7000 get first and second position in my bag. The D40 will be coming out with me more while the D70 would stay home.
Go with the D40 for casual stuff. The D50 only has the advantage when you use older lenses. Realistically what are the odds that your son will be swapping lenses?