The 24 month sizes are more baby-shaped, so they are wider around the trunk. 2T is for children who have started moving around, so they are leaner.
Children's is a word that denotes something that belongs to more than one child. There is no such word as childrens' because children is already plural and does not need an 'S' at the end.
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The word kids is used loosely to mean "children," but only loosely. Kids are baby goats, not baby people.
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