Colchester Zoo is one of the best zoo's in the UK. The amount spent on the place recently is quite evident. Yes, you have to walk around it (can't quite believe people have complained about that) but they have a huge array of animals and inquisitive children love it. There’s a “road train” which the kids love and plenty of picnicking areas.
If you’re local, consider the “Gold Card” scheme which gives free re-entry.
Can get very busy during school holidays.