“Wow, good for you!”

Your young child shows you a picture he drew of a flower. You want to encourage him, so you say:

Wow, good for you!

good for you

This phrase is used to praise small children. It means "you did a good job".

However, when this phrase is used toward an adult, it can seem insulting. It sounds patronizing, like you are suggesting that the person you say it toward is a little kid.