Pass the parcel for 28 children lasts forever. For that number I do two parcels, overwrap by 5 or so - i.e. Two parcels with 18 layers and get any other parents spotting to make it fair (plus few spare sweets just in case).
Doing knights and princesses with toddler activity group this afternoon. Going to do jumping beans style game, where call out person and they pretend to be that - princess, knight, king, dragon, servant, magician, jester. Doing jousting relay race, hobby horses (homemade), foam swords from 99p shop and some hoops to get. Then parachute game of dragons and rangers - dragons have to put eggs in forest (parachute), rangers have to get them off. Followed by sleeping beauty where all sleep under parachute. You could also do prince x says (simon says), what's the time mr dragon.
With a bouncy castle though they will probably just want to bounce, do you know how many allowed on at one time? How about having 4 activities which they rotate on, one of them being bouncy castle, one being crafty stuff, plus one or two more, so not all in one place.