How to make young toddler forget her parents left while I'm babysitting?

The little girl is a year and a half old, and we'll be playing and everything will be fine, but then she all of a sudden runs to the door and tries to open it like she's looking for her parents. The doorknob is low enough for her to turn it, but it's locked. She whines like she's about to cry and won't let me pick her up and bring her back to the play room. I have to push a button on one of her Fisher Price toys and it sings the ABC's. I'm just wondering if you have any ideas for how to keep her from doing that. She's met me a few times, but this is the second time I'll be babysitting her and the second time I've been to her house. She normally comes to play outside my house, but it'll be 7 pm and I don't want her outside.