Why does the SS call for Margot and not her parents?

Initially, the girls thought that Mr. Frank had been called up.... they envisioned concentration camps and understood why there was talk about going into hiding. The girls, of course, were wrong. The SS was calling up girls in order to remove them from their families and send them out on their own. This likely would have had dire consequences for the young ladies called up.

Margot is sixteen -apparently they want to send girls her age away on their own. But thank goodness she won't be going; Mother had said so herself, which must be what Father had meant when he talked to me about our going into hiding. Hiding. . . where would we hide?