Just spent a weekend in the Cotswolds. One of the things that DD, 5 and DS, 3 LOVED was a model village at Bourton on the Water (the actual village is nice too, almost to pretty to be real, though). It's nothing special but the DCs really enjoyed it. Went to Cotswold Wildlife Park a couple of years ago and it was fab.
Whatever you do don't go to the farm place at Wiggington! Its a sea of mud with some ducks on it. The Cotswold farm park is great for toddlers and much cheaper than the wildlife park. Bourton on the water is fab, especially birdland - the pengiuns have a special tank with a glass side so you can see them swimming under the water. Deddington is fairly close to Blenheim palace (in Woodstock) lots of parkland, little train, adventure playground, etc..