I know this is an old post but the subject still comes up often and just my two cents. For hand loaders it is an easy choice. Simply reduce loads in the 20 Ga until the child grows bigger. You will still have much more useful gun for him to grow up with. Even a single shot 12ga has little recoil if down loaded to 5/8 or 3/4 ounce and lower velocity and is still much better choice than a .410 for anything. I have pattern tested many loads including trying to work up a useful 30 yd duck load load for a friends 410. After that experience I vowed to never own a .410. A poor 20Ga load will still pattern much better than the best .410 load anything past 20 yds. The payloads is just too small. Going to smaller shot like 7 1/2 to increase pellet count also limits penetration and increases wounding on tougher game. # 5 shot is about the smallest shot that will blast through a duck to the vitals at 40 yds.and there are precious few #5s in a .410 shell. The .410 really is a experts gun, not a beginners gun . It takes skill you be proficient with such a small and thin shot pattern. The 20,Ga with light loads is still miles ahead of anything a .410 can do. My kids learned with a model 24, .22 over 20Ga and an old single shot Winchester 12ga with reduced loads. Occasionally I just bough cheap trap loads and opened the shells to change the #8 shot to a slightly smaller charge of #5 shot for hunting. The loads grow as they grow and no need to buy another gun. Some love the .410 but I am in agreement with many others that a .410, with its limited payload has the capacity to be a better wounder than hunter on anything bigger than close squirrels and small game birds.