Hi being a CM i also have varios seats for my mindees..i have to say the 31/2 year old i have usually still goes in my 5 point harness seat BUt if the baby is in it he goes in a booster seat with sides like this
www.argos.co.uk/static/Product/partNumber/3760921/Trail/searchtext%3ECAR+SEATS.htm
Someone quoted:
'not booster seat with normal seat belt until they are 4 yrs/15kgs'
But my mindee isn't and his paretns are fine with it AND they either pop him on a booster base or nothing which i obviously don't agree with but he is fine imo..i had an accident a while abck and he didnt budge one bit.
I realise the OP's child is younger but i think it alld depends on the size of the child as well as the age.