Our 4 year old uses a Safe n sound Hi Liner. We also bought a harness for it, but it was such a pain to do up and it didn't sit properly near his neck. He asked us how he was meant to sleep in his seat with the harness on. I did some online research and apparently, the harnesses that you add on, aren't much chop, as most people don't install or do them up properly and the one we got just irritated his neck. So, we just use the adult belt and his seat has an anti-submarining clip to stop it riding up his abdomen in the event of an accident.