MY almost 3 year old is also very tall and I just had to buy a new car seat with inbuilt harness, I purchased a baby love ezy combo. It has a very high head rest I think the biggest problem will be if your DD is too tall for the shoulder straps rather than the head rest being too short. It was cheaper than a maxi rider and almost as safe according to the online rating (I think it was done by the NRMA).