I would like to be able to carry either my nearly four year old or my nearly two year old, depending on which one of them is being tired/arsey, and have been told a hip seat is the thing to go for. It would be so noce not to have to take a buggy out sometimes - they will both walk most of the time now. Does anyone use a hip seat for children this old, and are they expensive, and is it really much easier than just carrying the child?