How do you say "to do" When asking a kid what he wants to do when he grows up?
Well, there's several way to say it. In addition to what you wrote, you can also use 做 or 當. For example, 長大後你想做什麼? or 你長大後想當什麼?

A more formal way, e.g. in essay questions, is to use 志向. For example: 你未來的志向是什麼?