Doesn't appear to be on Gunparts web site. 12 and 16 are the same, but they sell 20's separately.
If you're putting a Hogue on for the kid, give Hogue a call.
Plan "B" is a call to Mossberg's Canadian service guys to see if they have a youth stock. Both are in Ontario. Add the W's.
.mossberg.com/canada-customer-service