I understand your excitement but I would seriously consider starting with something not as loud. ie. a 22 or put a 22 conversion kit in it.
I started mine in my lap gun under their armpit and against my shoulder. and from the first time and every time. it's all about safety. safety on, aim, take safety off let them pull the trigger till the mag was dry, safety on and repeat. lol
I'm just getting my almost 6yo to the point where he can use the irons. bought him a savage rascal for his 4th b-day. tried irons with little to no luck. folded put on a red dot he does pretty good with that. some trips i'll take it off and see how he does with irons. still working on it but were getting there.
we took mom's centerpiece from thanks giving (a small pumpkin) and he shot the AR for the first time. the reaction was priceless! he pulled the trigger and was like WOAH! and just motionless in disbelief. lol he missed and wanted me to shoot it but it was so comical how serious he got after shooting it. I think he had a immediate respect of the power even though 5.56 may not be much to us.