I've always had the youngest in the middle. Oldest on the drivers side. I think they're less likely to jump out into traffic & I can tell them to climb over to the passenger/foot path side to get out when too busy. I also don't have to stand on the road side doing up seatbelts, because (for me) she can do that hersellf.

So with 4 I have DD1 in the front (no choice there), DD2 n the drivers side (no seat anymore), DD3 in the middle in a convertable seat & DS in a hipod on the passenger side.