If the toddler is over 15kg in weight I would suggest any of the High Back booster seats with an option to remove the back once the child is older. Most of these are for age range 4 to 11 but at 3 1/2 it is not worth getting any other sort as for this age they are on the small size, youngest had to be moved to a booster seat by the time he was 3 as the rigid sort was too small.
These seem to be pretty good