If you want to go somewhere a bit different and it is free for under 4's try
The Forbidden Corner - The Strangest Place In The World
I took my two a few times when they were younger and even though they are nearly adults they still talk about it saying what a magical place it is. In such beautiful surroundings too.
Be warned there are a couple of parts that small children may get a bit scared but with reassurance or avoidance it will be fine
You get a map when you enter and you have to try and discover all the hidden things and solve the riddles... to be honest I absolutely loved the place and would be happy to go back there without kids
And after saying all that I even broke my kneecap falling last time (don't ask!!) and still I love the place