How do you say "to do" When asking a kid what he wants to do when he grows up? The classic question (at least when speaking english) is to ask a child what he wants "to do" when he grows up. Now this "to do" is very vague - and, in a lot of instances, doesn't actually mean "do" (So much as "Be")
I'm interested, in Chinese, how is this classic question asked? 

Is it
"你长到以后想干什么啊？“  or is there  a more natural way to say this?
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?