but clearly he realises that 9yo girls don't have any boobs to speak of
Bollocks. (Not that he hasn't noticed, I've give him that, you clearly have a lovely lad)
but bollocks if you think 9yos don't have boobs. I can give you an example of my just turned 10yo dd with bigger breasts than me. And they certainly didn't just appear overnight.
Girls pubertal development begins with breast development and 2-3years later periods start. Some Yr6 girls get periods, some before that. Which means they would have had breast development start at 7/8years old. 7 is the cut off for precocious puberty, which means 7 onwards is normal.
Fortunately for dd the respect her primary school has for their intake means boys and girls do not change for PE together at any age.
It might not be a concern for you with a 12yo boy who's only just noticing these things, but it is certainly a concern to me with a 10yo who has been well aware of them for the past 3 years.
And it's a concern to the OP, whose dd is being tease for her choice of underwear.
Well kids are sometimes silly in maths so maybe that shouldn’t be happening either. what a ridiculous statement. Consider this: add 10 years to the children's age. At 17 kids, well boys, because this is about "boys being silly" being silly in maths is still boys being silly in maths.
At 17 boys making any comment on a girls underwear is sexual harassment.
Which one should you really be concerned about?
At what point does it transfer from being "silly" to "sexual harassment"?
When the girl feels uncomfortable? (Because OPs 7yo does, even if she doesn't fully understand why.)
Or when girls puberty starts? (Because that can be 7, or in some cases younger.)
Or when schools fail to adapt to the fact that puberty happens earlier than it used to 10-20 years ago, and therefore fail to safeguard girls (and boys) from harassment.
Why should it not be taken seriously because the girl is 7? Why should changing together be "just the way it's done" because it's convenience and staffing? Because that's not safeguarding children.