How stressful. I'm so sorry. I'd say parents need to educate their kids - and really, they should - but so many of the adults I see don't know the right way to approach a dog either. The difference is that most of them have the impulse control to stop and ask me - but they can't teach their kids what they haven't learned themselves.
It's a tough line to walk. I require my dog to handle herself around kids and likely couldn't keep a dog who wouldn't, but I live in a densely populated urban area with a ton of young families. I've had to learn a good, solid facade of ridiculous confidence; if I'm squared up and exuding authoritative energy, I can stop a kid in his/her tracks. I'm always surprised by how well it works, but something about me must make them not want to mess with me. It isn't about hating kids or being mean for fun. It's not mean. It's about using what you have in the moment to protect the safety of everyone involved.
Honestly, normally I can too. I work as a teacher, so being stern with children definitely isn't anything new to me. But I don't even understand what was going through this girl's head. She had zero regard for what I said or the attitude I was protruding - just 100% her desire to try and pet my dog despite what had happened previously. Absolutely bonkers to me.
I think with more experience and time I'll get much better with it as you are. Ryka has kind of always been seen in our family as the social, happy to interact with others kind of dog after she's done her sniffing and okay-ed them in her head. Just yesterday an old man wanted to pet her, and I was a bit wary because of what happened. But Ryka saw him, did her introductory sniff, and then wiggled her butt to demand pets and kisses after I gave permission. His wife came out to say hello, and Ryka wouldn't stop licking her face and nuzzling their hands.
So all I can think of is that really, my dog reacted based on my stress and because of how forward the girl was. My body and vocal signals were screaming, "don't come any closer", as were the vocalization of my sister-in-law, and because the girl didn't stop Ryka felt like she had to act on it. I don't want her to do that, so I've been really working on encouraging her to let me handle things since then. We have, however, realized that she's the kind of dog in sport that works harder when the handler comes at her more aggressively/head on while I'm with her, and only gives mediocre work when it's just general prey work. So I think it's a personality trait I need to be aware of and manage appropriately from here on out.
That's why I appreciate this forum so much. Lots to learn from lots of perspectives, and others' experiences are great learning tools.