Kodomo Keisatsu the Movie

Detectives in the special investigation team in Kanagawa go after the criminal organization "Red Venius," based out of Yokohama. While pursuing the gang, the detectives turn into children due to exposure to a special gas. Dekachou and the other detectives continue their investigation via an order from the National Police Agency. "Red Venius" then gives notice that they will assassinate the President of Tadzhikistan in Japan.
(Source: AsianWiki)