The best load will be 1 5/8 oz of TSS 9s through just about any full choke you screw into it. For a small kid, I would even consider something less than full choke to allow more wiggle room, and you will probably still be good to 40 yards or more. At least that is true with most hand loads of TSS which, until this year, have been the only option.
Apex Ammunition now sells TSS and their loads are essentially the same hand loads that the TSS gurus have been creating and testing for years and if I didn't hand load, that's where I'd go. But Federal has also announced that they will be selling TSS this year, but it hasn't hit the market yet so no one knows yet how they will perform.