CRAYON SHIN-CHAN: SHUURAI!! UCHUUJIN SHIRIRI

The film's story begins when Shiriri, an alien, arrives and turns Hiroshi and Misae into children. To turn them back, Shinnosuke must travel the length of Japan with Shiriri hidden in his butt. Meanwhile, Shiriri's father is moving forward with a huge conspiracy. (Source: Anime News Network) — Description from AniList community (anilist.co).