Child went berserk -- how to neutralize her?

She went bezerk because you didn't have the specific kind of workshop she was looking for. In the future, if you get a moody dwarf who's not claiming a workshop, build any workshop types you don't have yet. When you build the right one, they will know instantly and run and claim it immediately. (You can sometimes get a hint of what workshop they might be looking for by checking their preferences and skills.)

At this point, there is no way to bring her back. She will be berserk until she dies. She will refuse to eat or drink anything ever again.

You can try locking her in a room until the inevitable end, to keep her from aggravating your other dwarves. Or guide her to the caverns or a dangerous surface environment, if there is an exit close enough. You could also build a wall around her, and tear it down when it's no longer needed.

But, alas, there is no cure.