I need to move my 3.4 yo in to a new car seat so that his younger brother can move in to his. He is tall and over 15 kg but I am very reticent about just using a booster fastened with a seat belt at the moment. I'd like him in a harness for a while yet. Can anyone recommend a seat where I could use the harness until he's older and then remove it and just use as a booster? Thanks