How do you put a Sig brace on a Mare's Leg?
Given you're trying to teach children, this is the only time I will ever say it, but .22 AR pistol with Shockwave brace. You want them to understand how to line up iron sights, get a good cheek weld, etc. Normally I would say Ruger Charger with pistol brace, but the Charger doesn't have iron sights to speak of.
A .22 AR pistol would make for a really nice .22 tho. The shorter barrel is accurate, the velocity loss is negligible unless you're shooting hyper ammo like CCI Velocitor, and you can get good drop in triggers... maybe even a binary trigger? Then you can machine gun cans all day and not have to sell the house to pay for it!