I recently introduced my 4 and 6 year old daughters to shooting with an ultra-low powered BB gun. They both enjoyed their 1st experience and my 6 yr old announced shooting was her new favorite thing! Any way, the butt-to-trigger distance on the stock BB gun stock is about 14 inches. Even though both my daughter's are very tall for their ages, their length-of-pull distances are only about 6.5 - 7.5 inches. To start them shooting, I had them put the stock over the top/outside of their shoulder. Not ideal, but it was the only way I could teach shooting with an erect head.
When we came home, I made this semi-adjustable stock to fit both kids. I designed this stock for very quick l-o-p adjustments as the kids take turns shooting. It uses stackable stock length sections. Each section keys into the adjoining section with a pair of dowel pins and is held firmly together with strong magnets.
The stock isn't pretty, but my kids can fix that with some Disney stickers and magic markers.
*** stock pics in next post***