I see no one has replied to your thread and I think you have a good question, but I have no definite answer.
First I have to advise you to let the 13yr old kids become recoil comfortable. That age is when most children get over the fear of shooting a heavy recoiling gun and the AK isn't all that bad. My brother and I (along with any cousin that came to Grampa's house to shoot) were comfortable with a 12 gauge before the age of 10.
Back to the question: Just try it out. The AK is over gassed, so losing some of the force to a buffer in the stock shouldn't make it fiail to cycle.
However, if it was me I'd buy them a leather slip on recoil pad. (limbsaver or whatever) I actually use one with a chopped up Mossberg recoil plate to extend the length of my stock. I'm 6'3 + and it does wonders for speed, control and accuracy. A simple wrap around with some leather and "Goop" and you could make it permanent.
Also, tape some foam at the butt-plate and check weld, if they really can't handle it. That way you're AK still looks like an AK should!