I'm sure some of you have been here so I wanted to ask your advice . Ds2 now weighs more than 18kg and therefore should no longer be in the standard 9month-4y car seats but in a booster with a back on using the adult strap. We tried him in ds1's at the weekend and he loved it but (as I was anticipating) immediately slipped his arm through the strap so he was only effectively strapped across the waist. I know that Crelling do harnesses for car seats but they all say to be used in conjunction with the seat belt and I can't see from the pictures how they would stop ds2 slipping his arm out? Anyone with any experience of this or any suggestions.