I don't know about the children getting all the good parts, my son is only in year 1, and in his only play (last Christmas) he played the part of the star, which seemed pretty good. I don't know what he will be doing this year.
I did notice however, that at the last few school fairs (Christmas 2017, spring '18, summer '18) the PTA people seemed to win all the raffle prizes.
There were two massive raffles at the Christmas one, one was decent prizes like season tickets to the local soft play worth £50 or similar, vouchers for meals out etc. The other was a load of Christmas hampers, all stuffed with chocolates, wine and so on, probably all worth at least £50, some more. We'd bought about a tenner's worth of tickets and didn't win a thing. Fair enough these things happen. Except that I know a couple of the mums on the PTA. One of which I went to school with myself and she bullied me all through senior school. She won about 2 hampers and about 3 of the other types of prizes. Another PTA mum won a hamper and a voucher. Another PTA mum won two voucher type presents. I think out of the whole lot, there was about 4 people who weren't PTA mums who won something.
I am convinced it was a fix, and I have refused to buy tickets this year although the bitch I went to school with has left, her kids go to a different school now. We will see how this year goes before giving any more money to these things. I want to support the PTA and the school of course, but it makes it very hard when these things happen.