We've just got back from Gorran Haven nr St Austell. We found plenty to do round there (kids are 5 and 7 so around the same age as yours).
Beach when weather good: we went to Caerhays (nice big beach, rock pools); Gorran Haven (nice cove if you go past the main bit with the shop, but gets cut off at high tide) & Vault beach (v nice and quiet).
Pendennis castle in Falmouth had loads of activities running through school hols, we went to the Grand Medieval Joust which kids loved, also had minstrels, falconry & jesters - good day out!
Falmouth - we did the park and float (ferry) and the Maritime museum, which was surprisingly good & fish & chips from Rick Steins.
Eden Project - loved it. We went after 3pm so kids went free and adults got reduced entry (cost us £20 for the 4 of us). Was open till late (8pm, I think), so still had plenty of time to look around.
Padstow - bit of a trek (though drive to Falmouth was worse!) but worth it.
Charlestown - shipwreck centre and they had 2 big square rig ships there with pirates, etc.
Mevagissey - good for crabbing!
Heligan supposed to be good too, but couldn't fit that in. Lobbs Farm shop which is on site is v. nice!
DP and DS went to Fowey and had a 4h kayak trip which they loved.
On previous trips to Cornwall, we've been to Springfields & Lands End, St Ives and numerous other places I can't think of right now!!