here are a list of festivals on during the year. there are always things on, not sure what dates you're travelling.
as mentioned by Granard, fota is well worth a visit, much more impressive than a zoo, there's a lot of space for the animals.
i've heard a lot of things good things about this group, they do kayaking, even in cork city by night! lovely i'd say.
fitzgerald's park is a big park in the city, lovely flowers, water, a big playground etc.
near the park, a walk away, is this which is another option.
i love the crawford gallery which is right in the heart of the city and has a cafe run by one of the ballymaloe family. delicious.
speaking of which, if you're a foodie, ballymalloe is wonderful, but a drive away - look here
kinsale is sooo worth a day trip, you can get a bus from the city. i love the place. its by the sea so has lots of boats, water activities, quaint gorgeous craft shops, pony trekking, and it is particularly famous for its food. this restaurant is amazing, and there is a little fairground next to it for the children.
also in the city, the observatory is hugely popular with the young.
swimming pools, greyhound racicng, cycling, horse riding, go karting, cookery classes, art and sport activity days, cinema, toy shops, museums are all available in the city - just google for any specifics.
surfing is exploding in popularity, this is a good website to look at. the area is stunningly beautiful, but again involves a car to get there. i've seen gocar.ie, which do short term car rentals.
finally, not to be missed if you visit cork city is the english market. my link won't work, so you can google it.
yummy food, keep the dcs sweet with a shot of hot chocolate (excuse the pun )
i've left out lots i'm sure, but i hope this helps. cork was voted in the top ten cities to visit in 2010 by lonely planet so i hope you'll enjoy it .