Shrinking Pains, by E. M. Goldman
"Best friends Doug and Milo, both 12, along with Doug's worldly cousin Cassandra, 14, discover a fountain of youth; the water halves the age of whomever drinks it, but only for three hours. The boys enjoy shrinking to age six and going trick-or-treating, going to cinema at half prices and playing at the mall. Others know about the fountain, however, and will stop at nothing to get control of it."
Further regressions near the end, with a final twist for some characters.
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