I invited the whole class for ds2's birthday last year - he was in reception. Only 12 out of the 30 people came, all of whom had replied. I chased and chased, a couple of people had replied who weren't able to come but nothing from all the others .
We booked a local gymnastics club that is close to us - it was a sunday morning, there was nobody else there and they had the huge place to themselves. There were a couple of girls who looked after the group - did some games, some supervised stuff on the equipment in groups and then let them loose to play on the equipment as they wanted, supervising the bars and vaulting thing (can you tell it's been a long time since I did gymnastics!) but there was loads of other stuff - like massive soft play shapes to climb over, a long bouncy running thing (probably used as a run up by the proper gymnasts). They gathered the kids around them after doing the supervised group stuff to tell them that they could all go free over the equipment and the excitement on their faces was just pure joy - they did a count down to it, just 3-2-1 and by the time they said Go they were just fizzing and they all exploded away in different directions. Wish I'd caught it on camera because it was the sort of expressions you know they are so happy but I had been caught by another parent and so didn't have my camera at the ready unfortunately.
A mum had been to a similar party at another local gymnastics club and said that ours was much better - the others really weren't bothered about the kids, they didn't supervise for the free time. So it's worth asking around. I hadn't really come across gyms (for gymnasts rather than standard grown up work-out type gyms) before, particularly to have a party in, but it worked really well and wasn't that much more than hiring a village hall. Certainly less than hiring a village hall and an entertainer!
We've also done/been to parties at local sports centres which have been good. And one of the guys that runs football classes for after school clubs and does local lessons/holiday camp lessons will also organise a football party, so it's worth looking to see if there are any outside providers of after school clubs at your school to look at their websites to see if they also do parties. If they are from your school then you also have a feeling that the school must have vetted them and thus they are a known quantity, at least to somebody that you currently entrust your children to, which can be reassuring rather than just picking a name out of a book or off a website. Also ask around other mums to see if they have heard of anything or know good people. Somebody who is good at what they do can make a party fab for most kids whereas somebody who isn't can make even a favourite activity boring!