Halfords do a range that will take you right through from 9 months, the bonus is the shoulder straps adjust individually, so you can pull them to one side for a bigger child, but you'd need them to check the fit for you in both cars if your planning on swapping them around! As far as having the same seats to avoid arguments, pah, my lot have learned that I don't negotiate, they sit where I tell them to! Usually he or she most likely to be silly goes where I can see them most easily!