I started on an 870 12 gauge. On the rifle side, I’ve favored sub 50gr powder charges for the cartridges i shoot as a way to manage recoil. It’s not that I’m afraid of recoil, it’s just not something I enjoy, so I don’t chase it.
For kids, and like many of us in our youths, recoil is something that needs to be tolerated to a degree and managed when possible. I started shouting heavy 3” mags as a kid and I turned out just fine as a shooter. That said, I own the same compact beretta 391 with a 24” tube in 20 gauge as other have mentioned. My kids will shoot that first, but if I didn’t have it, it would be 870’s.
For about the last 10-15 years, I’ve shot an 870 20 gauge with a full length stock with a 21” youth barrel. I love that combo. I’ve said that if my kids start on an 870 20 youth, that they would have the youth stock with the 28” barrel I have for 2 reasons: longer for barrel for muzzle control and added weight to help reduce recoil.
At the end of the day, buy what you can afford and don’t look back. We’ve all had bad recoils moments in our lives. Those who quit because of it, weren’t cut out for it anyways.......haha. You fall down, you get back up.