Well OP - why didn't she just call him by his name? He does have a name - doesn't he?
We don't know the context at all. Maybe he was in a different class/year and she didn't know him. Maybe a new child had been told to speak to the school ecology rep (or whatever) about how to recycle her rubbish and didn't know anybody yet, so just asked the first person she met. Maybe she'd been told to speak to 'Tim, the school ecology rep' and asked somebody which one was Tim. However is the answer "It's Tim" to the question "Which one is Tim?" going to help.
TBH if this child is the only black child or boy in the school, all of the children would know his name so that story doesn’t hold anyway
Wow - Really???? Is that the norm where you're from - that a black person would be paraded around and introduced to everybody in the school/company/town as a huge point of interest, purely because they're black? I've never lived anywhere where that would have been a thing in any way.
All I'm getting from some of these replies is that being black is somehow a negative characteristic that you shouldn't ever mention, in case you expose a black person to the shame that they apparently should feel for being black. In the same way you'd say 'the man with the blond hair' or 'the tall lady" but would (I hope) not mention a negative or less than desirable characteristic such as "the fat woman" or 'the spotty lad'.
Surely, the best way to encourage and perpetuate racism is to make an issue of the fact that people have different skin colour and tell people that you mustn't make mention of it, in case they feel ashamed or embarrassed about having skin which happens to be a certain colour, as we all have.
Maybe if we could all be like children naturally are and just learn to observe that people have different skin/hair/eye colours, different heights, hair types, wear/don't wear glasses/hearing aids etc. etc. but, beyond using them as a simple descriptor to point out to a stranger which is the person they've asked for, not to care less about them, we could get over it and start working towards a racism-free world. Differences and diversity are not the problem - prejudice, discrimination and treating people differently because of them are.