Even though the duo are not kids, they got involved in kid-napping and were made to nap.
Btw, apparently Two-Mix is based on a real group, and they sang one of the anime opening. One of the member is Takayama Minami who appeared as herself in this case and is the VA of Conan.
That's why Mitsuhiko said her voice is similar to Conan. https://www.detectiveconanworld.com/wiki/Minami_Takayama
