As long as you ask and confirm, it doesn't sound like a problem especially if you plan to stay. I had a nanny show up to my daughter's 4th birthday at a park. She brought my daughter's classmate, and his 3 siblings. Before I could say anything or confirm she was going to stay, she left them all there! I was totally floored. It was now my job to keep an eye on these 4 kids w/ no parental supervision (EVERY other parent stayed, hello the kids were 4 yo) and I had my hands full already. I was not happy.