Don't eat too many sweets. You'll get cavities.
歯が痛いの？ ママに見せてごらん。あー! 虫歯ができてるよ。
Do you have a toothache? / Does your tooth hurt? Let me [mommy] see. Oh! You have a cavity.
I won't tell you to brush your teeth every time you eat something but at least, rinse your mouth with water after every meal.
Oh, his teeth are coming in! I didn't notice.
It seems like he's teething. I hope it doesn't hurt him.