Hawglips gave you good advice.. The choke I screw in my 870 20 gauge for kids is a 570 Undertaker.. It does not throw the densest patterns like 115 ish in a 10 inch at 40 but it is one of the most even 20 gauge patterns I have ever seen with both Hevi 13 7s and Fed HW 7s..
I would go with something in the 570 range with the H 13 7s for a kid.. It will give maximum number of pellets and the H 13s are very tame in recoil compared to other 20 gauge 3 inch offerings.