Van Hage Garden centre, Ware. They have a miniture train, about 60p per person per ride, they used to have an small animal bit, think its still there but we havent looked at that bit for a while, and they have a place that sells outdoor play equipment, and my kids normally play on it for a bit and so far nobody has complained. They also sell Wendy Houses and my 2 love playing in those 2. They also have a pretty good restaurant/cafe that sells decent kids food. And a lovely shop area to wander around. I have a 2 and 3 year old and they like it and kills a couple of hours.
Also can recommend Standalone, went there the other day, nice big area for them to run around, nice play equipement, mini tractors to ride around, animals, ducks etc. But take a picnic, it only has a basic cafe, but you can eat your food in the barns if the weather isnt good. Oh and also has another miniture railway, not one that you can ride on.
Woodside Animal Farm is also good, small but they have a lovely fair ground area, its on the way to Luton.
Fort Knebworth, never been, probably OK for your 6 year old, possibly OK for your 3 year old, no good for your 1 year old.