I initially heard, the first week or so, the father say on TV that they were sleeping in a room across the hall from him. I have no other confirmation than what he said that it is true.
That would fit from the layout posted. One would have to cross past the top entrance of the stairs to gain access to the master bedroom. This might have been difficult for MC to do IF Elizabeth were sitting on the lower step putting her shoes on and the abductor were facing Elizabeth, thus facing the top of the stairs, making MC need to wait for fear of detection, until after they left the house, which was probably momentarily.