My 20 month DS (average size, always hovered around 50th percentile), progressed smoothly from size 1 to 5. He has been in size 5 nappy pants since before his first birthday I think. There are offers on at the moment so i'd like to stock up. I'd hopefully like to start potty training a few months after he turns two. Is it worth buying size 6 in advance or do some toddlers never reach size 6?