J.P.
I don't know why there isn't a 15mos. in children's sizes. Children all differ so much in sizes that I guess it's easier to go by an average size of children each particular age. For your daughter though, I would go with a 12 mos. in pants, 12-18mos. in shirts, and if you're going to get a one piece I'd get the next size up. The nice thing is that if they're too big, eventually she will grow into them. Pants can be rolled up, or down at the waist if necessary, and shirts can be rolled up at the sleeves or tucked in.