ORIGINAL: buckmaster68
I have a remington 870 in a 20 guage that my kids have used with 3 inch shells and it doesn't seem to kick too bad. It is very tolerable for a younger kid and shouldn't make them gun shy. Also make sure you use a full or extra full choke.
Just my two cents.
My daughter has the same setup. With a primos tight wad choke and #6 rem hevishot it'll kill them out to 40+ yards. But we like them in closer than that. The best out there? I don't know. But so far it's been a very good gun for her.