DS is tall and heavy (weight 16.5kg at 21 months, height wise has grown out of his group 1 car seat). As far as I can tell every 5 point harness group 1/2/3 seat or cushion style seat (like kiddy), recommend switching to use the seat as a high backed booster with the normal adult seat belt when child weighs 18kg; but looks like this might be before he is 2, and they are supposed to last until age 4. Is it safe to do this with a 2 year old, and if not what am I supposed to do (help!)?