I live in the Brighton Area also, I had my childrens party at Domino's Farms. It was really nice, they give you a house and kids get to use the farm fully, as well as pony rides. We did it in December, it was chilly, but the barn was warm and the house was really nice. They were good to work with and very accomidating. It includes a hay ride, as well as pizza, water and soda, all for around 250.00.
Kensington Farm also will do kids parties, I have not had it there, but have been to one, it was nice.
Hope this helps!