I like the idea of a youth 20 gauge. It could serve two purposes. Kid dear gun and home protection.
I think you have found your answer. A good youth sized 20 guage pump will have a shortened length of pull for the stock as well as a 20"-22" barrel that accepts removable chokes. 5-6 rounds of 20 guage buckshot would make the gun a potent attack stopper. Plus if you have kids (or a smallish wife) they can use it for hunting or target shooting. Incidently a youth 20 guage is on my list of things to buy but I am not in a hurry. My son is only 7 years old so it will be a couple more years before I introduce him to shotguns and my wife is most comfortable with a handgun.
By the way OP, while I think a youth 20 guage is a good choice do not automatically rule out handguns. I would not consider a pump shotgun to be any easier for a novice to master.