Whipsnade every time.
Agree with others that Twycross is a bit manky. We are closer to twycross but would rather go to Whipsnade.
What about Woburn Abbey? We've had some lovely days there?
If you're in Leicestershire Twin Lakes at Melton Mowbray is fab too. A little run down in a couple of places but my ds's favourite ploace in the world. (he's 5)
Have a great though and hope the weather is nice for you!