The zoo is better value pound for pound, and is really for all ages. You can do that in a few hours or take the whole day.
Thanks for the replies so far.
We have a c. 3 hour window so the Zoo wouldn't be a runner.
Would we have enough time to do the London Eye - we should be arriving in Liverpool Street by tube @ c.11 AM and need to be back in Covent Garden/Holburn tube station by c.2:00 PM.
Sim