I’d say it can be a less common form of the latter. The Free Dictionary has an entry for it.
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thytisSo, it is "buckle up an adult" and "buckle in a child"? "Buckle up a child" and "buckle in an adult" are wrong?No, that doesn’t sound quite right to me. In its transitive use, buckle up or buckle in is used usually for children, at least where seat belts are concerned, anyway.