I'd ask them to put the softplay stuff away and just have the bouncy castle.
DD had a leisure centre party for her 4th so in a sports hall with bouncy castle, some of those hoppy rubbery animal things they sit on and bounce around, a shit load of balloons thrown around everywhere and a bit we set up with some mats and butlins style dance music (agadoo, witch doctor...).
The kids were very happy and entertained themselves to the point they didn't want to leave.
2hrs is the sweet spot.
1 Hr running around like crazy things
30 mins for food and singing happy birthday
Final 30 mins of running around while you clean off the tables, cut up cake and get it in party bags.
They all start filtering out and you load the car with leftovers, gifts and a tired slightly hyper, possibly over emotional child.
Keep the food simple, the kids won't be that interested in it, it's just fuel and something that's stopping them from playing. Ideally keep it all vegetarian because that way you don't have to stop the vegetarian/halal/Muslim/Jewish child accidentally getting a ham sandwich.
Sandwiches - jam or cheese in quarters.
Veg - cherry tomatoes cut in half, mini cucumbers or cucumber rounds
Fruit - berries, melon slices
Crisps - big box of different flavours, spread the bags down the table so they can grab a bag
Biscuits - party rings, chocolate fingers, Oreo's
Cakes - mini rolls, mr Kipling, fairy cakes
Couple of plastic jugs filled with squash. More paper cups than you need because they put them down all over the place and mix them up.
Assuming parents will stay, either make enough of the above so parents can snack or buy in a box of nicer biscuits and some bottles of schloer or something along the lines. You don't need to cater properly for adults but having a drink and a biscuit available is nice.
When prepping you will need the following
- cups
- plates
- cake knife
- napkins - for faces/hands and for wrapping cake for party bags
- blu tack
- duck tape (always find a use for this, like sticking down table cloths or taping a bin bag to the end of the table for ease of clean up)
- bin bags - for rubbish and for transporting lots of bulky weird shaped presents (don't mix them up)
- kitchen roll - at least 2 children will knock over a drink
- zip lock bags for left overs
- party bags and a few spares for the inevitable siblings who tag along with no notice