I too have the Rem 700 with the short, threaded barrel. Mighty nice with a suppressor.
For a small, youth gun, the Ruger American is chambered in 300BLK. I think there is a Predator version and I think they are all threaded. Had the Ruger been available when I bought mine, I probably would have bought it. It would be lighter and more compact. You can easily find a Encore in 300BLK if you want a single shot with traditional wood stock and forearm. If you're not going to suppress the 300BLK or use in the AR platform that matches your 223 setup with a simple barrel change, you might be better served by a 30-30, 308 or traditional caliber. Where the 300BLK shines is subsonic, in my opinion. It tosses big, heavy bullets at a slow pace that are easily suppressed. My 2 platforms are an AR which is sighted in at 50 yards and the Rem 700 at 100 yds. Beyond that, the drop is substantial. If I wanted a longer range gun, then I'd use my 308. If I wanted a 100 yd rifle only with easy kick, I'd probably think about things mentioned above like the 30-30.
However, I see where the Ruger American is now in 6.5 Creedmoor... distance and little recoil... hmmmm...