In an ideal world, of course the school and any other adults involved, should make sure that children are included and children are kept safe - but sadly that doesn't always happen.
If a child is being physically attacked and hurt, or bullied by another child, it wouldn't be fair on the victim if the person who hurts them was invited to the party - but equally, it would be u fair to exclude the child because the adults around them have not stepped up and intervened appropriately.
So what is the parent of the party host child supposed to do. They could have small parties, but is it right to tell a child they can't have a big party because they have been hurt/bullied/victimised by another child, and despite this they are supposed to put the other child ahead of themselves?
Is it right that a child should have their birthday spoiled by having the child who bullies them/hurts them there? It would mean putting your own child's happiness second to that of another child, who has already made them unhappy and hurt them. I don't know that I could have done that, if we had been in that situation.
I was the bullies' victim, all through the last year of junior school and into senior school, and they made me suicidal by the time I was 14. I don't knkw if any of the bullies had SN or difficult family backgrounds - but even in my mid-teens, I was obviously not a good enough person, because there is no way I would have I vited any of them to any party of mine. But luckily for them, my mum didn't believe in parties for children over 8, so it was never an issue. Sadly she also didn't believe in taking action when your dd tells you she's being bullied, or supporting her or backing her up at all.