Parents of girls ages 4-6?

That sounds like a reasonable price and a super party. Like others said, that price should include a certain number of guests with further charge for each guest beyond that. I would also say a specific time allotment, too, maybe 2 hours, and charge beyond that. (Not that the party would have to end, but you would be done with your activities and pictures and would leave.) I assume you'd charge for whatever pictures they order. You'll have to think about that. Good luck with your business.