Hatfield Park Farm recently which is located in the grounds of Hatfield House in Hertfordshire. It is a wonderful place to take children of all ages to see the different breeds of sheep, ducks, geese, pigs, cattle, turkeys, chickens and donkeys that they have on the farm. They also have a beautiful playground area.
Entrance was reasonable at £4.50 each per adult and £2.00 for a child ticket (0-3 go free) and for 50p you can buy one bag of feed to give to the different animals. The animals are friendly and you can feed them at the various designated feeding points. I would recommend getting at least two bags of feed or at least one per child as it's hard not to feed every animal you see as they come running to the feed stations as you get closer to them.
There are about four sandpits darted around the farm where the kids (and, um, adults) can play in the sand and use the toy cranes to move the sand around and build sandcastles. This was a big hit with my little one!
When my daughter went to the farm a few months back there were four piglets which are now much bigger so it was lovely for her to see them grow into big piggies.
There were quite a few friends and families having picnics and birthday parties around the area as it really is beautiful and the children are kept entertained in a safe and secure environment which I love.