You shouldn't smile so hard because it makes your face look wrinkly, and it will leave wrinkle marks on your face.
この場合、次のように言うと良いと思いますよ。
ーYou shouldn't smile so hard because it makes your face look wrinkly, and it will leave wrinkle marks on your face.
「そんなに思いっきり笑うと顔がしわしわになって元に戻らなくなるよ」
to smile so hard で「思いっきり笑う」
to make one's face look wrinkly で「顔がしわしわに見える」
to leave wrinkle marks on one's face で「顔にシワが残ったままになる」=「シワが元に戻らなくなる」
ご参考まで!
Your smile is adorable, but you shouldn't wrinkle up your face like that. You won't be able to get away from having wrinkles on your face permanently.
ご質問ありがとうございます。
"Your smile is adorable,"=「あなたの笑顔は素敵です、」
"but you shouldn't wrinkle up your face like that."=「でも、そのように顔をくしゃくしゃにしてはいけない。」
"You won't be able to get away from having wrinkles on your face permanently."=「永久にしわが取れなくなってしまうことになります。」
ご参考までに。