Just a couple of bin bags and expanding foam is enough. Line the seat and cockpit area with a big/thin plastic sheet (DIY store decorating sheet is fine), in case you mix too much foam. Put a pad on the seat to raise young'un to a reasonable height/depth, mix foam and put in double bag, put on top of pad(s) and sit them in the seat until it hardens. Remove bag(s), reshape with knife as needed, then wrap in Duck tape. You can also fill in the depressions left by the seat pad with more foam and re-trim as needed.