You're going to get paint in your mouth if you leave your mouth open like that.
You better close your mouth otherwise you're going to get paint inside it.
この場合、次のような言い方ができますよ。
ーYou're going to get paint in your mouth if you leave your mouth open like that.
「そんなふうに口開けてたら口にペンキが入っちゃうよ」
ーYou better close your mouth otherwise you're going to get paint inside it.
「口閉じないと、口の中にペンキが入っちゃうよ」
ご参考まで！
アメリカ合衆国 
カナダ 
