うちは refers to a period of time in contrast to some other period of time. The contrasting period of time is represented by the modifier in front of うちは.
During a visit with Anzu’s family, Makoto receives a request to babysit for another witch. Things are fine at first, but after a while, rain begins to pour inside. Anzu’s mother realizes that the child they’re watching over has rain magic.
杏子の母:
「小さいうちはコントロールできないんだろうね。不安になると今みたいに降らせちゃうんだね」
“You don't have control while you're small, right? When he's uneasy, it pours like just now.”
真琴:
「なるほど。そうですよね。いきなり知らない所じゃ心細いですよ。我慢してたんですね」
“I see. So that's it. It's a little scary suddenly being in a place you don't know. You've been patient, haven't you?”
