Ok, you want an answer as to wether her it is fair that all children benefit from the funds raised at a PTA event even if they don't attend.
The answer is yes, of course that is fair. PTAs are charities. They have a defined set of beneficiaries, and the beneficiaries are the pupils of the school that the PTA is associated with. You would not be able to have a PTA if the funds didn't benefit all the children in the school, you know that, and you are being obtuse by banging on with that question.
Apart from anything else, the disco is one event. I would have thought that the PTA you are on does more than one event a year. Just because a family isn't contribute to one thing, oes not mean they will not contribute to another. You couldnt keep track of every thing that every family did or didn't support anyway.
You didn't like my coat analogy, fair enough, as like you said, you would still want extra sibling to cover the cost of the disco you just don't think that you should make more profit from one family just because they have more than one child. So change my example to someone going into a shop to buy three coats for their three children. Should the shop only sell one coat at their normal retail price and then sell the two additional coats at cost? So that they are being paid for the materials but not making a profit on coats two and three?
Of course not. That would be silly. But it is the same as what you are suggesting. Charities do have to run like businesses. They even have to have business plans. If you can't see that the two things are the same then you need to open your mind a little and try to see something that you don't want to admit.
You also need to come to an agreement with the rest of the PTA about what the main aim of your disco is. If the aim is to raise money, then your idea really does defeat the point. If the aim is to have a disco so the children who want to go have some fun, then suggest your idea. If your PTA is clear that it wants to do both, then it can do that by charging full price to everyone and allowing parents to make their own descison about whether their children go, as they will anyway.