Clean the rim with isopropanol and apply 720° of tape. Put a tube in the tire, and seat the tire and tube with full inflation for 24 hours. Then carefully take the tube out, breaking only one bead off the rim if possible, and insert the tubeless valve. Inflate. If it holds air, add sealant, inflate to max pressure, swirl it around both horizontally and vertically, then have the kid ride the bike hard. Hopefully at that point it will do the right thing.