This thread showcases AB at its worst. The poster, in an earlier thread of theirs, made a reply about "lefties wriggling" in their responses. What we have here is righties doing their own bodily, and indeed moral, contortions in order to swerve what was very clearly an act of blatant racism. We even have accusations of the man handling it badly, being the cause of the whole problem, and that - heaven help us - he should have been grateful and thanked her. It absolutely beggars belief. Regardless of all the dancing-around going on here, she thought that a black man with two white children was wrong and must be challenged. In this thread, we even have the notion that if it were a white man with two black children, also in a public place surrounded by CCTV, that too should rightly be challenged. Heaven forbid anyone, adult or child, should be in the presence of an adult or children of a different skin colour, as it must mean something is wrong. What a horrible, soulless, shrivelled way of thinking to start with; to then calmly justify it, and to then further make the man the villain of the piece and berate him for his lack of gratitude, makes my skin crawl. Would the same challenge be 'justified' if the parties were white/asian, Welsh/Polish, or any other combination you can think of?
As suggested earlier, I also think a female guardian would likely not have aroused "suspicions". Add in the possible different ethnicities of a female babysitter, and it starts to look like we need a detailed set of guidelines on when and when not to challenge.
There's no suggestion the children were distressed-looking at the time; if they weren't, I'd suggest they may well have become distressed by their guardian then being challenged by a stranger.