Hi Ruggles. As you can tell from my username, I'm a fan of Cornwall. Have you been before/do you know it? How far are you coming?
To break up the travelling in/out of Cornwall, there's two EXCELLENT places near Launceston/Okehampton not far off the A30:
Hidden Valley - indoor and outdoor puzzles/challenges/clues to follow, which are quite challenging really and gets adults involved. Family ticket around £40-50.
Tamar Otter and Wildlife Centre - brill if your kids are into animals and like to get up close to feed/hold birds/watch feedings. Family tickets £20-25.
You're unfortunately on the wrong side of the estuary for the Camel Trail, but I'd guess that the ferry takes bikes - print off tide times before you go. The Camel Trail is seriously seriously busy with bikes between Padstow and Wadebridge. Quieter beyond, but there's a lot of traffic in Wadebridge to tackle with your 7yo.
Our DCs love the short speedboat rides from Padstow. You'll see them - cheesy names and shark paintings on them. No need to book, but be prepared to hang around a little if you go at a busy time. Less than £10pp I think. If you want a longer sea life watching trip, defo defo book!
Good, big beaches, I'd drive down the coast to Constantine, Trayarnon, Mawgan Porth etc. If you want calmer water (which I always think is warmer - probably just psycological) drive to the south coast for the day. Glly (gllyanvase full name I think) beach in Falmouth is one of our family faves. They also do SUP lessons there at WeSup. Great little company, excellent eco ethos, have your ££ ready, but I think that's the same the county over if you want lessons - be ready for £30+ pp for lessons, c.£10 for hire. Book lessons in advance.
You should probably hit up Port Issac (looks beautiful, I've not been) and Tintagel.
Enjoy it!