My son is almost three & he fits comfortably in a size two Fred Bare t-shirt and mostly wears size three otherwise, so I'd say they might be a little on the larger size. I'd get the 000 that way it might be a bit big, but still fit at the same time

As for size one fitting a one year old, don't count on it! Today my Alexzander is wearing a size 0 pair of shorts!!! Admittedly if he was wearing a nappy they wouldn't fit properly, but even with a nappy he was wearing size ones at two years old. It really depends on the build of the child & whether they are in nappies or undies when they get bigger IMO. I'm struggling finder winter pants this year because he needs the size two length, but they fall off him!