Trigger! So many guns marketed for juniors have abysmal triggers. HW30 is one as are some of the BSA's and the Webley MKI Vulcan wasn't great either. Why is it all right for a beginner when we more experienced shots wouldn't touch something with a poor trigger?
Size and weight are the next consideration. Stocks are too long for young people. Anything "man sized" is never going to work and anything over 7lbs will never be lifted.
I have a BSA Cadet with a shortened stock and its the business for a starter gun. Even the trigger is smooth and breaks at about 3lbs. Power isn't important.
A Webley Vulcan MKI with the trigger worked on is the next size up. A ASI paratrooper works too, as does a Buccaneer. The Crosman Rabbitstopper has a too long a shock but the right weight. Its a tricky one, thats for sure.
If you can get youngsters plinking you will have them hooked. But please have something they can actually work and a half decent trigger. Dare I say it even an airsoft small gun might be more fun. Youngsters are well aware that they might have to wait to grow into a full sized rifle.
I would say, though never owned one, that a BSA Ultra PCP with an even shorter stock might work really well. I'm sure someone could add to this.