Maybe you need to go somewhere after work then? Then you have more of an excuse because you are actually going somewhere. You seem like you don't want to hurt her feelings? Once she has left lock the door and don't answer it. She has a stroller to change him in or can lay him on the grass. There is no need to come back to your house.

Seriously if she is the last one just say we'll have a good night and walk away! She can show herself out. If she follows you then you need to have a serious talk with her.