I have held 2 birthday parties so far.
1 for 55 kids and 1 for 30 kids. Both were very chaotic. For the 55 kid one I had a bouncy castle (nightmare unless you can get an adult to man the bouncy castle at all times and make sure too many don't get on all at once. We actually had a nose bleed due to having too many on at once). I also had an animal man which was £90 for an hour (Fantastic, however I think 55 kids was too many. They lost interest because they were having to wait too long to get a turn at touching/holding an animal.)
For the 30 kid one we had a dance class party in a dance studio. This was £75 for an hour in the dance studio and an hour in the food area. My DS really enjoyed himself but a lot of the boys wouldn't join in because "dancing is for girls" A disco with a DJ probably would have been better.
If you plan to have a pinata (always a hit with kids) make sure they all line up to take their turn and stay in the line. I tried to do this but then all the kids got out of the line and crowded round the pinata. How we didn't have another accident I don't know and I felt it was quite dangerous. Also the kids were all fighting over the sweets so what I would do next time is put in 30 party bags closed with elastic bands with some sweets, 1 x punchball balloon and 1 x bottle of bubbles so there's one bag each rather than putting the sweets all in loose and some kids getting loads and others having none. 99p shop do 8 x punch ball balloons for 99p and poundland do 9 x bubbles for £1.
For decorations and plates, bowls, etc pound shop, 99p shop, Asda and Wilkos are really good. Don't bother with cups as you always get loads of spillages. I get those calypso type cartons that you poke a straw in the top. Also so you don't get loads of wastage put the food out on the plates ready and cling film the top. I do cheese sandwiches, crisps, raisins, fruit (you can get little fruit pots or grapes and strawberries are quite easy) I give out the savoury plates first then when everyone has finished eating do Happy Birthday and candles. Then I give them a bowl to go and get their pudding. I did donuts on a cupcake stand (you get 12 for £2 in Asda), ice lollies (63p for 8 in Asda) and a piece of birthday cake. An idea we have had for next year so we don't even have to make sandwiches is to have a make your own wrap station. So we will go round and give each child a wrap, some lettuce, cucumber, tomatoes, grated cheese, ham, chicken, mayo etc.
Next year we plan to have a multi sports party. The local secondary school hires out it's school gym plus a room for food for £42.50 for 2 hours in the gym and 30 minutes in the food area. They provide the sports equipment so you can either do the games yourself or do what I will do which is hire a sports coach for £50. You can get 8 x medals for 99p in the 99p shop to give out at the end.
A big hit at both parties was hiring a mascot. We have had Mickey Mouse and Jake and the Neverland Pirates. It was £25 for 30 mins including a gift and the kids loved dancing with them each time.