For beaches in North Cornwall I would recommend Daymer Bay (great for rockpooling), Watergate Bay, Polzeath is good but very busy. Constantine, Mawgan Porth, Trevone are all fantastic beaches as well.
Cycling on the Camel Trail from Wadebridge to Padstow, or Wadebridge to Bodmin is a good day out. In Padstow you can go on the speed boats which the kids will enjoy, eat pasties & ice cream, buy fudge, visit the Rick Stein Deli etc. If you follow the trail towards Bodmin you can stop at the Borough Arms en route for some refreshment or the tea rooms at Nanstallon.
Hidden Valley in Launceston is meant to be a good day out for older children.
There's a small cinema in Wadebridge ideal for the rainy days and also Waterworld in Newquay.
Other suggestions are: St Kew Golf Course, Tennis Courts & Park in Wadebridge, lots of geocaching locally, walking the coastal path, kayaking (Camel Canoe & Kayak), Crealy.