From teaching my son to flyfish when he was about that age [Leave your rod at home unless he insists you take it. If he does then do.]

Try to get him/her "his" rod.
Should be something like a 7 1/2 or 8' 4 or 5 wgt.
SLOWLY show him how to make a backcast and forward cast SEPERATELY. Let the backcast drop to the ground.
When he can get 15-20 feet of line out [including the leader] take him to a bluegill pond with a few foam spiders and woolies. Let him pick the first fly to use.

Turn him loose.

Keep some of his fish so he can show his mother. Put some back. Explain why. It would be neat if you have some kind of container at home so he can keep the fish a day or two to show his friends. Cook a few of his fish no matter how small.

Stay as long as he wants but do leave when he is ready to leave.

It was 20 some years ago when I did that and we still fish together several times a summer!

Donald