We went to a children's farm today (Ponderosa), it was very close to us.
It cost us £16.50 for a family of 5 in and that price was for all day.
There was plenty to see an do, you can handle and feed animals too
Plenty of picnic places also a restaurant.
A few playground areas dotted around too. Plenty of hand wash points too.
Would recommend

Some photos in my gallery section
http://i193.photobucket.com/albums/z144/dbc2612/001.jpg