Anything chambered .357 and up should have enough stopping power for those playground firefights, but if you want him to be totally safe, I would recommend a Thompson .45 with a 50 round mag. He probably won't need ranges longer than 50 yards, and at full auto, he can take out multiple targets quickly. As long as you teach him about muzzle rise the recoil is very easy to handle
I think they are making a "Spongebob" color scheme for the thompson now.... should be a big hit with your nephew
