A bit further away but Surrey Docks Farm in Rotherhithe (SE16) is perfect. We had a 1st birthday there. Clean private room that has the farm yard on one side and the river on the other at ?50 per hour, but you won't need to provide additional entertainment with all the animals around. For your age group you could also provide donkey rides. Farm manager's name is Barry. He was rather busy and difficult to pin down for details but well worth it.