Have you considered scaling back the party to something you can afford? There is no way you can watch 10 children in a huge venue like a zoo, with all the hazards. It's already an overstimulating environment for young children, and then you add in the birthday, the gifts, whatever cake you are having, and having them separate from their parents. I think $27.50 per child is extremely expensive for 4 year olds anyway!
Children this age need some small play space (like a family room or back yard), extremely limited entertainment (they are not able to follow a lot of games, for example), and many of them don't even separate all that well from their parents except in the daily routine of a preschool - and even then, some of them don't do well. If they all want to run off to see different zoo animals and the parents are reduced to saying, "Stay here. Don't go look at zoo animals," it's not going to be too much fun for anyone.
I think there is great pressure on parents to "make memories" by holding parties at big venues, and then each successive parent has to "out-do" the last one by finding a new place. The bounce places and the McDonald's play spaces and the Chuck E. Cheeses and the zoos make a lot of money off this reality, but it's really not good for young kids or for the parents.
We followed the "age rule" (5 years old, 5 guests) and held parties at home until our son was about 9. Even then, it was candlepin bowling and 6 kids plus 2 adults to supervise. Everyone had a wonderful time, every time. Simple decor, simple "theme" (like putting out all his dinosaurs on the table), and simple games or crafts but with the emphasis on running around.
I think you have to give the type of party you can afford. You really can't go to people and say, "I'm paying for a party where you and I both know that I can't supervise your children, but you have to help subsidize it by paying your own way." No matter what your intent, that's how it will appear. For parents with several children, this sort of thing really adds up. I don't think you'll be pleased over the years when you keep getting hit up for birthday gifts and admission charges.