They will have lovely time.
Based on ds's 4th birthday party at home, plus others:
I make an initial plea - please DON'T have pass the parcel. It is a nightmare with 20, very hard to keep track of the layers when packing, and even though children think they want it, it is always horrible with children sitting down, glumly acquisitive, utter misery. Hell for the helpers too and you need lots of helpers for it at this age.
Have lots and lots of helpers on the bouncy castle. With 20, things are absolutely bound to get hairy at times. Intervene early.
Make tea a major feature - kids much nicer once they have been fed.
Sorry can't remember from thread whether parents are staying. Try to feed them, or at least give them tea and biscuits.
Sorry to say this too but have a rain plan. Plan the schedule of things and have it pinned up - you don't have to stick to it, but it helps.
I'd agree What's the Time Mr Wolf is great but make sure that everyone who wants to gets a chance to be Wolf.
Games that were brilliant at ds's:
Kim's game (maybe 8 items on a tray, they all look, cover the tray, remove one, remove cover, what's been taken? Go through all items.)
Make huge blanket den for all children, then read them a story, with another adult doing sound effects. Massive success when they were tired towards end.
Who's Next? Circle of children. Give each child a card with pic of an animal (plus name of animal, unless you can draw very well!) Cards held up facing inwards. First child makes their own animal noise, then somebody else's. Then that child makes their animal noise and somebody else's, etc etc. Worked quite well.