I can't recommend the Cotswold Farm Park highly enough.
Not only do you get farm animals but also tractor rides, bouncing pillows, adventure playground, wildlife walks, a maze, rare breeds and the chance to bottle-feed lambs, hold teeny ducklings/chicks and today, we saw twin lambs being born - wow!
You also get that tasty Adam Henson from Countryfile whose farm it is (though sadly you don't get to hold him...). Everything is included in the entry price, no nasty extra charges for rides, there is a lovely cafe and shop, plenty of free parking and it is in the middle of the most beautiful countryside.
It was the first day of the season today, and my ds and I were the first customers, we bought a season ticket which allows entry all season for about the price of 5 visits. Oh, and there is a campsite too if you are that way inclined. My ds would live there, given the chance.
It's really great.