Why would you want to invest money building a custom rifle for youngsters when there are so many sub zero, over the counter, available rifles. A very simple choice would be a 6.5 Creedmoor because it can handle anything on 4 feet walking in North America with proper shot placement. Recoil is very manageable for small shooters and, for the cost, you can get each one their own. Buy one that is easy to restock and they can still use it when they get older. Then, build each one of them a rifle, if you still love them when they turn 18!