Yep - what TDRILL said.

I know it will make you grind your teeth - but give up on the cross-chain thing. The idea is for the kid to enjoy cycling - not to have someone continually bothering him with stuff he doesn't understand. Let him run up and down the rear cassette - if he runs out of gears he can treat the front ring as a last resort extra-big change perhaps. He'll work out "using the gears nicely" for himself when he's ready. Enjoy your time with him - in all likelihood your fast approaching those few years where he won't want to be seen with you at all.