Make sure you tell mommy as soon as you go poop, otherwise your bum will get sore.
You have to tell mommy right away after you go poop. If you don't, you'll have a sore bum.
you do poo ではなく you go poo や you go poop と言います。
as soon as you go poo / poop
ーMake sure you tell mommy as soon as you go poop, otherwise your bum will get sore.
「うんちしたらすぐママに言ってね。じゃないとお尻が痛くなっちゃうよ。」
bum「お尻」
get sore 「痛くなる」
ーYou have to tell mommy right away after you go poop. If you don't, you'll have a sore bum.
「うんちしたらすぐママに言わなきゃダメだよ。言わないとお尻が痛くなっちゃうよ。」
as soon as の代わりに right away「すぐに」を使っても表現できます。
have a sore bum「お尻が痛い」
ご参考まで！
カナダ 
