Urgh, a bit more info OP might help!
When are you going?
Age of kids?
Budget?
Have you got an itinerary in mind?
Are you a sporty or sit on arse type family?
Do you thrive with noise, crowds and adrenaline rushes? Or do you prefer calm quiet activities away from the madding crowd?
If it's summer you are going the learning to surf might be an option, Newquay is busy vial has pact vibrant beaches and sometimes dangerous good waves.
Gweek has some lovely walks, and the seal sanctuary which is great for younger kids.
Flambards is fun for school age teens and older primary school.
St.Ives is arty and has a Tate gallery, which displays the St. Ives artist movement, Barbara Hepworth and some other geezer famous artists.
There have been numerous films and TV programs made in Cornwall in recent years, so if you are in to that you could plan to visit various TV locations across the county, Godolphin was a back drop for Poldark, the house is owned by the NT, it's a little gem and never feels over crowded as it's slightly inland, but the gardens are stunning, it has a very ethereal feel about it, with large mossy boulders, and stunning broad leaved woods, with a wealth of bird song.
If you are an avid reader you could visit the Jamaica Inn on Bodmin it's totally urgh but great for families or Fowey (which has charming book shops...or did I haven't been for years) and independent coffee shops.
Truro has a reasonable shopping centre and a comparatively modern (compared to medieval) cathedral.
Charles town is a historic dock, so if maritime history is your thing it's worth a gander. Nr to Charles town is the Eden project, worth allowing a day to do everything.
National trust properties worth a visit, Lanhydrock, you can hire cycles, large play ground, great walks, stunning gardens, interesting house.
Cotehele on the banks of the Tamar. Good walking, stunning garden, small house, but old clipper you can visit at the dockside and a pleasant flat walk leads to the mill.
St Michael's mount is cool, trelisik and Anthony are worth a visit, but check opening times, as Anthony isn't open every day.
The lost garden of Heligan is fascinating, if you like gardens.
Minack theatre is worth a visit, even if you are not seeing a performance.
If music is your thing have a look and see if the 'Fisherman's friends' are playing (they sell out fast), but sometimes do impromptu performances in their home town, some pubs have live gigs.
If you have a budding engineer / scientist then the Marconi station should be on your list and Goonhilly (Earth station) was once open to the public, then closed, then rumoured to reopen, but I don't know if it has reopened.
I have been to Cornwall for several years, so my info, is probably out of date, but would check out the relevant websites for up to date info.