Wa Alaikum Salam Sister @sualeha Masha Allah. It was so nice of you to give your daughter a surprise birthday. I pray to Allah Almighty that Allah Almighty gives your daughter a long life and gives her good fortune.
You made a secret plan with your son. That we will surprise him. So I was very pleased to know.
You mentioned that your daughter was studying in the room and was moody. So when you gave her a surprise, she was happy and thanked you guys. So this is exactly your point. When a person suddenly gets happiness, even if he is sad, he is very happy. After that, your gift is also looking very good.
Best Wishes 😍 Always be happy
Regards!