i said on the last page my ds1 was terrified of a little boy, i didnt exclude him or avoid them, i explained to my child that this little boy was young and learning like him. and i supervised and helped my son to try and play nicely with him. stepped in when needed and pointed out the fun they had together when they did manage to play nicely. as they got older they were actually good friends, but from 2-4 yrs was a hard phase, but we went to the same toddler groups nad saw each other regularly, so imade the choice that it was worth working at, to help ds1 deal with it, i accepted his feelings and spoke to him about them, offered him reassurance and made sure i was close by.
the op cannot do that at the pre-school, but she needs to speak to the staff and find out what is going on and what they are doing to deal with it.
this party could be the ideal opportunity to hopefully see the nice side of this little boy. i would assume his parents would stay, but even if not at a party you supervise and deal with issues. i would make sure there were extra adults on hand, appoint someone to shadow the little boy and hopefully all lcan have a nice time and the ops dd can see that actually this little boy isnt as scary as she thinks.
i would also point out that at the age of 3, children can be 'terrified' of lots of things, part of our job as aparent it to help them understand these fears and overcome them as necessary.
having a party where you invite everyone bar one child is NOT the done thing, like it or not, it is rude and hurtful at times.