I have the same problem, but worse.
I live in a townhouse community lots of neighbors, very close, and lots of kids. On Saturday I about lost it after I was interrupted every 5min. I bet my door bell rang 15 times in one day. I had two neighbors just walk right into my house. All I wanted to do was clean house and relax. I have kids ring our bell at 9am that is way to early in my opinion. Then they don't juse ring it once they ring it continuously. If we barbeque outside and eat at our picnic table the kids will come over and ask for food, stare at us, and tell us how good it looks. I will tell them they need to go home, etc. They refuse to listen and keep starring at us eat. I've just come to the conclusion people are just rude these days. And hardly anyone teaches their children manners.
What works sometimes for me is to just be consistant. I would say something like.
"We eat dinner every night at 5:30, you need to go home and when my daughter is done eating if she wants to, she will come and get you."
Try being polite but very very direct.
If she is still sitting on your porch and doesn't listen to you go back out and say
"Excuse me would you please go home, sitting on our porch with our daughters toys is a distraction and she won't eat her dinner."
It sounds a bit grown up but at 9 if that doesn't work go talk to her parents. Or start ignoring the doorbell.
I like the green/red flag idea another poster recommended I might have to try that.