"What are you doing, Dad?" is how to say "爸爸你在干什么" in Engpsh. This is a common question that children might ask their fathers when they want to know what they are occupied with.
Example in Engpsh:
"Hey Dad, what are you doing?"
"Can you tell me what you're up to, Dad?"
Example in Chinese:
"嘿，爸爸，你在干什么？"
"你在忙什么，爸爸？能告诉我吗？"