D.H.
We have used Grisham Farms and loved them. In fact we loved them so much that we are using them again this year. They are located in McKinney and I know that you can do on site birthday parties. We had them come to our home two years ago. They brought 10 animals and did an educational presentation for the kids for my son's 2nd birthday. We LOVED it. My son is turning 4 in February and he wants them again. I highly recommend them. We've always had them come to our house for the party but I know they do parties on site.
We've also been to a party at the Frank Buck Zoo in Gainsville and it was fabulous. It's a much smaller zoo that we love and go to often in the spring and summer and I think it's perfect for young children because it is so much smaller.
There's also always the Fort Worth Zoo as well but I believe they book up pretty quickly.
Finally there is The Heard Museum in McKinney. We looked into having my daughters party there when they had the wonderful moving life sized dinosaur exhibit there. I'm not sure how many animals they have there on a regular basis though.
Oh and there is also Sharkarosa - they have some fabulous animals and a bucket train ride. I think that they do on site birthday parties but you'd have to check their website.
Please let me know of you have any questions. You can find each places website if you google their names and locations.
D.