You haven't said what the family like doing, but...a family ticket for Tintagel Castle is under £15
http://www.english-heritage.org.uk/server/show/nav.15393?utm_source=website&utm_medium=flashmap&utm_campaign=romance
http://www.cornwall-online.co.uk/english-heritage/tintagel.htm

and the Merlin's cave is right there too
http://www.thisisnorthcornwall.co.uk/king_arthur.html

...there's a lot of legend that makes it all more fascinating to visit.

You might want to check out some of the links down the left-hand side of this page for more on the surrounding area
http://www.thisisnorthcornwall.com/tintagel.html

I haven't been yet, but I'd love the visit the Eden Project
http://www.edenproject.com/visiting-eden/index.php

Go see some seals?
http://www.sealsanctuary.co.uk/

Some fantastic beaches to explore
http://www.visitboscastleandtintagel.com/site/attractions-and-activities/beaches-and-coves

Junior surfing, if the kids might be into that?
http://www.adventure-centre.org/juniorsurfing.html

Just got back from 4 days in Cornwall. Not been there for 30 years, things have changed!

To be honest we thought Tintagel was a bit of a dump and didn't stay long. However Port Isaac was very pretty and is where they film Doc Martin so lots of familiar things to see.

Boscastle was also quaint and remarkable how it has been rebuilt after the disaster. We took breakfast at the Riverside cafe. The poached smoked haddock and eggs were sublime.

If you are going to try Padstow then drive to Rock and park at the furthest point in the town then get the ferry across (£3 per adult return). Padstow is quite pretty is a little busy but full of seasidy type shops only half of which are owned by Rick Stein! Speaking of whom, try out his fish and chip shop at the far end of the harbour. I had the Plaice and mrs Hi De Hi had the Haddock. Both were very tasty and cooked in beef dripping.

Go to Godrevy and park in the National Trust car park then walk up to the head of the point. There is an isolated beach below where the seals bask. You will need a zoom lens on the camera and if you have binoculars take them.

There is a huge surfers beach at Portreath. The cafe in the car park does an excellent breakfast. Full english for me, Mrs Hi De Hi had the scrambled eggs and smoked salmon which she said was brill (and big portions). It opens at 10am but fills up quickly with surfers so get there at opening time.

The Tinners Arms at Zennor does a nice pint of local beer and very good food.

Cape Cornwall is pretty wild (or it was yesterday) Take a walk round the point to the volunteer coastwatch station. The watcher will let you in and show you how the radar tracks the ships and how all the technology works. He might even let you take a look through his huge binoculars. It is a voluntary organisation and you drop a few coins in the box as you leave. The Ice cream booth in the car park also does a nice line in Apple crumble ice cream (and other flavours).

On the South coast Fowey is very picturesque St Austell isn't worth a visit although the kids would probably like the Eden project (not my cup of tea but very popular with families).

I should add take plenty of loose change as almost all car parks are pay and display (found a free one in Lostwithiel) and always take odd amounts like £1.10 per hour and don't give change if you have to overpay!

I would think about going to the South Coast - Fowey is a beautiful spot - lovely town with nice place to eat, beach, fantastic coastal walks, shops and a good location for visiting the many attractions and fishing villages Cornwall has to offer.