They will get a lot more use out of the .22 and probably have way more fun. The .20 gauge would be good to introduce them to bird hunting with. I had a gun of both calibers given to me at birth, and have been shooting them since I was 4 or 5 years old. Before I was legally able to buy a firearm I only used the .20 gauge maybe twice (bird hunting), but shot the .22 all the time. It is also way cheaper, as already stated. 500rds for $15 vs. 25rds for $5 for the 20 gauge.