Lexington does not have a zoo, but there is the Salato Wildlife Education Center in Frankfort that kids would enjoy.
You could take them to Gattitown one night for pizza and games.
The Kentucky Horse Park is an option if you think the kids would like to be outside and see some horses up close.
There is ice skating at the Lexington Ice Center.
As far as restaurants go.... ones that might be more kid friendly, but are good local faves....
Joe Bologna's (Italian, good prices, downtown in a historic church building)
Ramsey's (Local, regional food...very casual, inexpensive...southern cooking)
Cielito Lindo (Mexican, very kid friendly laid back atmosphere, good food, downtown...can't tear or really up anything)
I hope you have a great trip and let me know if I can be of more help.