Is "watch yourself" eventhough it means be careful, used more in nagative situations or nagatively? So, if someone says that to a child, the person is more about criticing the child than caring?
"Watch yourself" has a more negative air about it. It isn't something that I would use kindly. It has a connotation of almost sassiness, and so could even be considered rude in some contexts.
A kinder way of saying something similar to a child would be "Watch that you don't get hurt!" or "Take care."