Try doing the process of sending her to boarding school again. Have the parent do it. Maybe it just didn't take the first time.
If the child don't go to boarding school you can delete her with the move objects on cheat. Her picture will be grayed out then. But save your game before you do it to be on te safe side.
I don't think deleting the child is a good idea. Unless you've got some kind of new information, the child will disappear if deleted from the game, not become grayed out and at boarding school.
OK will try that , and when will she come back ? when she turns into teen ?
Yes, when she turns teen, you'll get a notice about it. You can then decide if she comes home or stays in boarding school until she's a young adult. Also, be aware that you cannot choose traits when she ages up if she's at boarding school. But she'll come home with some skill points, which helps make up for it.