When someone is angry they can be attractive. When we find someone attractive, we call them cute. By combining these two ideas, we can call someone cute when they are mad. This is usually for people we know well such as a child, family member or someone we are in a relationship with.
Even when he's playing up, he's still a little cutie!
Despite being in a foul mood at times, my child is still a sweetie!
If you wish to state that even when your child is angry they are still cute, then you may implement the example phrase given.
Sweetie/cutie = someone (especially a small person) who is attractive and endearing
When a child gets angry they are sometimes funny and cute with the way they act
if someone is mad you can also call this 'angry' so you could say
'they are so cute when they are mad/angry'
子どもが怒ると、行動がおかしかったりかわいいときがありますね。
怒ることは 'mad' の他に 'angry' とも言えます。ですから、次のように言えます。
'They are so cute when they are mad/angry'
（怒るとすごくかわいいです）
"You are so adorable when you are angry" This phrase uses a synonym for cute. "Adorable" is another word you can use for cute.
Both phrases suggest that when your child gets angry, they appear cute to you.
"You are so adorable when you are angry"（あなたは怒るとかわいい）
このフレーズでは "Cute" の同義語を使っています。"Adorable" も "Cute"（かわいい）という意味で使えます。
どちらのフレーズも「あなた（＝子ども）が怒っているところを見ているとかわいい」という意味合いです。
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