I'm with clover if it's physically doable. The safest place is the centre seat so I would always have the youngest there. In the middle you can't take a direct hit and the youngest will be less likely to withstand a direct hit if in an accident. The second safest position is behind the passenger seat. So I would try to have the second youngest there, and oldest behind driver. That's all for best case scenario in an accident. Obviously due to what seats you have they may not even install properly in that order. So however they install the best is the safest in that case. It's like a big jig saw puzzle some times!