I've RTFT and just wanted to add my voice to those who say they would leave.
No way would I have my dd take a slot in a session for a group of people she is not part of.
If we had travelled a long distance, we would hunt Pokemon hang about in a cafe or something until the session was over, or we would go back another day.
If we had missed the signs and dd had just gotten in, I'd apologise and I'd bring her straight out and do as above.
My dd would not bother anyone, or sit and stare and she would keep herself to herself but even then she would still be brought out and we'd wait around til 7pm or return another day.
I wouldn't use others peoples children as a learning opportunity either, there's a shit load of info online that can be used to educate dc to be aware of the needs of others, there's no need to take up space at a special needs session.
My dd is very much aware that others have needs that she doesn't, that her day to day life is much much much easier than it is for others and that things like disabled car parks, wheelchair spaces on buses, accessible toilets etc are not there for her benefit and that it's wrong to use them, because there's a good chance it would prevent someone who needs to access them from doing so. That sometimes she has to wait because others need x y or z before she does.
I've talked to her from a young age as things arise, often about things we've read on these threads, when we see someone abusing a facility I'll tell her why it's wrong, we talk about things in the news or TV, and she will do little things like push wheelie bins from middle of pavements to the side in case a wheelchair user can't pass and challenges her peers if they use certain words as an insult.
I asked dd this morning what she thinks is the right thing to do if a sn session started/was in progress when she arrived and she said leave, that it's not a session for her so it would be rude to stay. She said it's extremely unfair that the session only enables the children who need the sessions to access the place for two measly hours a month, and that they wouldn't even be allowed that if people who shouldn't be there just stayed anyway. That it must make parents angry when their child's tiny amount of time is taken from them.
Sorry to all the parents and carers on here facing shitty attitudes day after day after day after day. 