The safest route is the single shot and it is the most economical.
Next step up is a pump. I would just like to add that even the full size 870 Express has a very short LOP - 13 3/4" but if you can buy a stock extention kit for $12 which comes w two 1/4" and one 1/2" extentions that can be used individually or in combination. This is w the synthetic stock.
The tactical extended stock is a great idea as long as it has or can add a quality recoil pad.
The Ultimate Option: Rem 1100 -20 ga. Much softer on the shoulder, easy to shoot well with and when you crush targets, well, don't we all Love that...
Problem: It is also the most dangerous option, and you must train them to load one at a time until you trust them to load more.
Deeper Pockets - Graco makes an adj butt plate w LOP adj. So both you and the kids can use the same gun. Move the pad in for them and then out for you.