Suggests that MK probably saw them the second time when she was near the stairs, and the other two were by the front door.
That's my guess, too. What doesn't make sense is the waiting for 2 + hours to tell the parents. Elizabeth may have sat on the stairs to put on her shoes. So, why not wait for the them to leave, then run to the parent's room. Scared or not. Get back behind a wall and just wait a few minutes then take another peek to see if the coast is clear. That's what I did when I was 8 and we had strange goings on at our house one night. This waiting 2 + hours just doesn't make sense.