If they have been doing fine with irons so far, try them in the woods with irons. Young eyes are amazingly good at using peeps and other irons. With little instruction and ample instruction they will have fun and get pretty good ... as you already mentioned.
Or, as was already suggested, make sure the scope power is low (no higher than 4 anyway) . Both eyes open is also great.
But practice and time is the key.
You are in a great spot anyway, with eager kids who already have the safety bug going for them.
Good luck with those young hunters. Kudo' to you for helping your kids and the sport.