Here's a controversial statement: recoil sucks.
In the short term it hurts your shooting, in the long term, it hurts your body.
Here's a more surprising statement: one of the worst offending guns in terms of recoil is the 20 gauge youth pump.
Youth guns aren't brutal, like 3 ½-inch 12s are, but they kick much harder than you would expect. I see it every year when we start high school trap practice. Boys and girls shooting light 20 gauge pumps like 870s and Mossberg 500s are the always among the ones pulling their heads off the stock or snatching the gun away from their shoulder as they pull the trigger. Both are subconscious efforts to get away from a gun that is hurting them and both can persist as bad habits even after they switch to lighter kicking guns.
I think recoil velocity is to blame. Recoil velocity is the measure of how fast a gun moves backward under recoil. Youth pumps loaded with 7/8 ounce target loads have about the same foot-pounds of recoil as 12 gauge with a slightly heavier load but the little guns come back faster. They can give you a nasty slap when they go off that's more painful than the same amount of recoil energy hitting you more slowly in a heavier gun.
Well-meaning adults naturally assume that a little gun and light 20 gauge shells won't kick much but they can be painful. A good way to prove that to yourself is to shoot one from your off-side shoulder.
I waited until my kids were 11 or 12 then started them with youth 1100s which weigh a ton, are gas operated and kick not at all. They are great for duck and turkey hunting where the young shooter doesn't have to carry the gun around. When we went pheasant hunting I never brought my own gun and we would stop for breaks so I could hold the gun while kids rested their arms.
There is nothing wrong with youth pumps. They are cheap, light and a safe action type. Kids like the shucking action because they've seen it on TV. My younger son insisted on switching from his 1100 to an 870 Express Jr. the minute that gun came into the house.* Just realize that youth guns can pound young shooters and they will rarely admit a gun hurts them. It's up to you as an adult to limit practice sessions, seek out good light loads like Fiocchi trainers and Winchester Feathers, and to make sure your son or daughter has the most painless introduction to shotgunning possible.
*I took it back from him when he outgrew it and sometimes shoot turkeys with it.