Several places have already been mentioned (Seal Sanctuary, Cheney Mill, etc.). I'd particularly recommend Cheney Mill. My kids would practically move in there if they could!
Flambards is fun (apparently - haven't been myself for about, ahem, 25 years , but my parents took my dds at Easter and they enjoyed it).
Trevarno is a garden which has a really nice adventure playground for the kids.
Land's End is really tacky.
Cornish Cyder Farm is quite good fun (and free). 2-y-o might like tractor rides.
It's a bit further away, but we've also enjoyed Lappa Valley Steam Railway. You get the train and then there are really nice play areas and things at the other end.
The maritime museum in Falmouth is good, although your dcs are probably a bit young for it really (though some exhibits appeal to all ages).
You can get the boat from Truro to Falmouth.
A nice beach for small kids is in Hayle. If you go up past the church (Phillack) and keep going and park at the end of the road (near The Bluff pub), the beach there has lots of little shallow pools (except at high tide) which are safe for los to paddle in.
Buy local paper (e.g. The Cornishman or The West Briton) for tide times, and things on while you're there.
Also in Hayle is Paradise Park. (Mostly birds, but some animals, also train ride and play equipment. I think that it now has an indoor soft-play area too.)
Tehidy Woods are nice for a walk.
Ummmm.... running out of ideas now. Will re-post if I think of anything else. You'll have a great time, anyway.