Affirm meaning and the child’s power
Let’s say you respond with, “How wonderful! What an ambitious goal! You could be part of exploring space! Becoming an astronaut is going to take a lot of work. Just think of what you can learn! You will learn everything necessary to explore space. That will be exciting!”
This response will hopefully accomplish two outcomes. First, affirm the core meaning of the child’s hope. Second, emphasize the potential for being successful with the hard work of achieving the goal.