Last time we were down that way we joined the NT and stopped off at Stonehenge on the way back for free too. It seemed like a decent place for a stretch on the long drive - but it looks like you need to book in advance now. Lands End or The Lizard to take the kids to the end of the country. Screech owl sanctuary - although its 20 years since I went there. If you want to go a bit further, Dartmoor or Paignton/Torquay - which had a decent zoo. @royston tyke - Looks like you might still need to book in advance for a lot of attractions, so you might want to do a bit of forward planning
I'd do Lizard instead of Lands End. Lands End is pretty bland and they charge a fortune just to park up. I think there is some token theme park there to try and justify the obscenity of it all. There are some stunning, beautiful and dramatic areas of Cornwall that are worth plenty of time and effort to see. Lands End just isn't one of them sadly.
Tell you somewhere else that our kids loved at that age - Crealey, near Exeter. Think it's developed since we went, but we camped, and you get free access to the theme park, which is targeted at younger kids. Hours of entertainment and not to packed. https://www.crealy.co.uk/ Think we combined it with a trip to Newquay once, and also with the SIdmouth Folk Festival another time.