AIO not wanting my neighbor's kids to use my bathroom?

I am friendly with many of my neighbors, and one family in particular I am closer to that live a few doors down. It started a few years ago when I fixed their broken lawnmower when the husband was traveling, and I'm friendly with both of them and their three kids. It's mostly just the friendly neighbor kind of chatting when we're both outside, me getting some plant cuttings she had been growing, etc. I haven't been inside their house (except their garage) nor have they been in mine, until now.

The other day, one of the daughters in this family, I think she's about 8 or 9, rang my doorbell and asked to use my bathroom and said it was an emergency. I didn't really know what to do so I let her in. I know kids can sometimes have to go very bad and I didn't want her to have an accident on her way home, but I didn't really feel comfortable as a single man having a young girl in my house without her parents knowing. It would have been fine if her parents had asked me to babysit, I've done that before for friends and it's no problem, but I didn't like having a child just invite herself into my house, even if I understand why she asked.

I did bring that up with her mom but she didn't really seem concerned about it. AIO?