I went to Drayton Manor last week for the day and had a lovely day with DD who is 5. There is a zoo at Drayton Manor as well, but it is quite small. Thomas Land was ok but DD was very keen to go on a lot of the older rides but she was too small, Thomas Land was a bit tame for her. There weren't many queues either, but then we did go on a rainy Wednesday.
Twycross Zoo i absobloodylutely love! There are a lot of monkeys though. I haven't been for a couple of years and the last time I went they were doing quite a lot of redevelopment, which is now finished.
So I think what I'm saying is, both are good fun. Twycross is probably a cheaper day out though but then children love a theme park don't they. You won't go wrong with either