The Healey Cider Farm, and nearby (1/2 mile down the hill) the Callestick Ice Cream Farm (both with animals), Gardens up this way inc Trelissick and Trewithen, Bosvigo in the middle of Truro......
cheapish but good food and kid friendly, the Hawkins Arms, Zelah (just off A30) and Adam and Alex have two young boys 8 and 6....(one of my locals).
There's a play centre in Truro , just off Kenwyn St, if you are up here but not great....
The National Maritime Museum, Falmouth, more for the 8 year old.....Raise the Roof Penryn/Falmouth (climbing frames and wait for it, a café)
A full day trip could include Trerice Gardens and nearby Dairyland (A3058 to Newquay - not far from the Hawkins but also the Miners Arms at Mithian - and St Agnes is nice to visit - great beach just to the south at Porth Chapel.
In St Ives, there is a really nice leisure centre on the top of the town (inc swimming pool)