DD is about average height, but very light (just over 17 kg) and from what I can see, the recommendation is to keep children in a 5-point harness until they are 18kg. We're going to have to move DC3 into a Group 1 car seat soon (i.e. hopefully into DD's Britax Renaissance seat), but I'm wondering if it's a bit OTT to find another 5-point harness one for DD. Should I just go for a high back booster that uses a seat-belt?