I would be speaking to the Head about her/his commitment to inclusion.
Either 'The Unit' is part of the school, in which case, all the children are involved in a whole school trip, or it's not, in which case, it's not an 'inclusion unit', it's an exclusion unit!
Of course they should all go- it's a pantomime, ffs, not bloody Hamlet! What's the worst your DD will do, OP? Need to be taken out for a while to chill out? Then they could take a pop up tent along, with some familiar sensory toys and ask if they could put it up in the foyer, or else they could take her for a little wander round the outside of the theatre and have a look at the cars, or the shop windows. With a 1-1, there shouldn't be much of a problem, should there?
If she's likely to bolt, they need to ask you what you would do with her-do you still use a backpack with reins? Would you object to using them? I was still using them with DS at that age as he was a bolter (did I say was
?)
The school should have gone through all this and decided what strategies each child will need before agreeing to the trip. Instead, they have taken the easy way out and asked parents to keep the 'difficult' children at home. I would not be making it that easy for them if I were you!