We recently took a complete family trip to the Zoo, including the grandparents. Everyone had a lovely time, there is something there for everyone. The littlies especially enjoyed the seal show.
I would suggest, like at most family attractions that you pack lunch and only buy treats, food can be very pricey.
Wear good walking shoes, there is a lot to see, and probably take a stroller for anyone 5 or under, my 4 year old really wanted to sit in a stroller by the end of the day even though she never uses one.
All in all, highly suggest a visit to the Zoo, it is a fantastic family adventure.