I have been looking at car seats all day and now I am totally and completely confused. If they have a convertible booster can they stay in the harness? Or once their shoulders hit the top of the change to booster marker is it safest to move them to the lap-sash belt. Or is it a combination of this plus a weight rating? If you have to move them once they hit the booster marker it seems odd to have such a young child in a lap sash belt only (DS is almost at the middle of the change to booster marker in DD's seat and isn't 3 yet).