Adults often talk to children about what they will be like as an adult. To talk about what a child will be like in the future, use the phrase "grow up to be ___":
I want you to grow up to be healthy and happy.
I don't want him to grow up to be some kind of weird, awkward loner.
Do you remember that time when you said that you wanted to grow up to be a comedian?
Another way to talk about a child's future is to say "be ___ when you grow up". You mostly use this to talk about a chid's future job:
What do you want to be when you grow up?