I've got a seven year old and personally I wouldn't put him in the back with the lap seatbelts, he always goes in the front. If you stop and they fly forward it will be dangerous as there's no support around the chest and shoulder and it's going to hurt them across the stomach area. As for changing the setup, I don't think you can as there's no anchor point for a seatbelt over the shoulder in a cab, I guess that's why they are lap belts in the first place.