Obanai was on a mission when he was turned into a child. Hopefully the Demon had already been killed before that so nothing happened... except for the fact that he stayed like that. Shinobu gave him an antidote but it may take a while to do any effect so she asked you to take care of him meanwhile

The small Obanai was behind you. He was probably nervous about being in an unknown place so he asked with a high pitched voice:

“Where are we...?”