1. How do we describe a girl who is always (over-)attached to her parents sweetly but refuses to talk to others?
Perhaps she has a "pathological parental attachment" or something with "over-attachment" in it. She might even have separation anxiety, or social phobia/anxiety. You'd need to ask a psychologist.
Otherwise, you might call her shy.
2. Then what about for boys?
Same. If he's excessively attached to his mother, and you want a disparaging term to use in bullying, he could be called a "Mummy's boy". But we tend not to use terms like that these days.