Dreamscomingtrue (and any other white people who don't get why this doesn't work the same way in reverse)
The long-term, centuries-long impact of racist colonialism that deliberately destroyed the infrastructure, economics, politics, culture, societies and individuals' lives means that when a white person is racist towards an "other" (BME or person of color, depending which terminology you use) there's a shit-load of historical racial abuse coming along with that word. That just doesn't happen in the other direction.
Of course, it can be intimidating to any child to suddenly be the focus of stranger's attention, and that's why many people (in a wide variety of cultures, but not all) have the 'don't point/stare/comment, it's rude' lesson that they teach to children. But that's NOT the same thing as a white teenager deliberately calling a beautiful young child a mongrel.
I haven't heard people using that insult for decades now. I genuinely hoped that it had just lapsed, as it's such a deliberately insulting word. And just by having heard that word, the OP is then left in a crappy situation. IF she turns around and objects, she's suddenly 'that angry woman' and the situation may escalate. If she doesn't, she's just had that label placed on her, and those children. Neither of those are ways she wants to have be seen - either by the perpetrators, or the children she was with.
That's why racism is a crime, and a hate crime as well. Even one word of it.