There have been times when adding a child from new FamilySearch to RootsMagic that FamilySearch does not send a complete packet of information on the child. The record received from FamilySearch gives only the sex of the child and assigns it a record number in RootsMagic. This child can not be added to nor deleted. This is what we call a "Phantom Child".
To remove this "Phantom Child" select File > Database Tools from the menu and run the "Clean Phantom Records" command.