baby face
顾名思义，baby face 指的是“可爱的娃娃脸”。

Jessie looks younger than she really is because she has a baby face.
Jessie 看起来比实际要小，因为她长了张娃娃脸。

straight face 
表示“没有表情的脸”，但多指“强忍住笑声或情绪，装出一副正经的样子”。

It was hard for Annie to keep a straight face.
Annie 很难忍住不笑。