Detective Conan OVA 03: Conan and Heiji and the Vanished Boy (2003)

Conan and Ran are visiting Heiji and Kazuha in Osaka when the four come across a trio of kids while touring the city. These children, who look eerily similar to the Detective Boys but with different personalities, are looking for their friend who has gone missing—and the two young detectives agree to help find the vanished boy.