This has always been my DD's lot in life, unfortunately. It's only now that she's in her mid-teens that she looks anything like "normative" in terms of size.
So you've got a lot of years of this ahead of you, I'm afraid.
You've already correctly surmised that the more YOU say to mitigate the situation, in all probability, the more patronizing and obnoxious others will become over including her.
Here's what we found helpful--
* Phone contact for "prospecting" (NOT in-person visits)-- and in that initial contact, state student's GRADE level... then, if that goes well, the child's CHRONOLOGICAL AGE... and leave size out of it. They'll be surprised when they meet in person, but at least they will already be thinking something more positive than they'll think otherwise. KWIM?
* Let your child handle it, as much as possible. Yes, this is going to mean some meltdowns and tears-- try to step in JUST before that point, when possible (so that there isn't the risk of being labeled as a maturity issue when the real problem is frustration with being utterly ignored in the face of higher level inquiry and desire to learn). We have SO, so, so BTDT. I eventually was able to signal to DD with a head shake when she should just give it up for the time being-- that is, when the adult leading the learning activity was just NOT going to treat her as a genuine participant no matter how much she tried to be "good."
* My tactic with large group settings led by an adult volunteer/docent like that? Wait until the lag just prior to a transition, and gently encourage your child to try again when the adult has a free moment. Yes, this does mean that your child's raised hand DURING the presentations isn't going to mean anything. But it's still the lesser of evils. If there is time, it often pays to do that at the tail end of the previous group rotation-- then the person KNOWS that your kid is the real deal, and will treat him/her accordingly, as often as not.
I hope that one of these ideas is helpful. I know how frustrating it is to pay for opportunities that SHOULD be beneficial, and find that they are simply exercises in additional frustration and alienation instead, when your child gets treated like a younger sibling who has tagged along as contraband and is probably just going to be disruptive.
