Have a conversation with her. I think people get it in their minds that we're just always here and don't mind "because we love children". They have zero concept that we have a life beyond our job since it's at home.
Tell/text/email her that you "felt very disrespected yesterday that you did not respect my closing time, nor even ask if it was ok if you pick your child up well past closing time. Yes the weather was bad but you didn't even leave work until _____. From here on out ______ must be picked up by 5pm at the very latest so that I may get on with my evening with my family. Thank you for your cooperation."