Well we had 35 kids last year and their parent too(as the children were 3/4 they didn't drop off. I didn't really want the parents to have to do anything) I wanted them to relax a bit so I hired one.
It was brilliant/he was super entertaining for parents too (as he wasn't naff) and I just sat back and faffed with the food/cake (my fave bit)
If the children were older I'd just take lots of games/raquets/beach balls etc and turf them out in the garden with an inflatable.
We used a guy who was amazing I wouldn't hesitate to recommend.