It is inevitable, unavoidable. But not the end of the world. A bit of rain is refreshing, replenishing, life-giving. But not necessarily what you want on your one holiday of the year. It doesn't have to mean your holiday home becomes a prison, little faces pressed against waterfall windows and the dog whimpering at the stable door.
You're in a self catering cottage, so chances are you'll need to be cooking. Brush up your skills in style and learn how to make the most of that bought-fresh-from-the-fishing-boat mackerel, with a class at Rick Stein's Cookery School in Padstow.
Get a bit creative and make your own souvenirs from a family holiday to remember. This county is an artists' Mecca with the likes of the Newlyn School of Art (which does day classes so you can get all 'Stanhope Forbes'), near Penzance. Or if your brush skills aren't up to it, go crafty at the Threads Craft Workshops in Hayle, where children's classes are available too.
If the children need something a little more active, then a halfway step to getting outdoors could be an indoor riding school like Lakefield Equestrian Centre in Camelford. Set on a farm, there are indoor and outdoor schools, so practice inside until the sun breaks through and you can trot off on a hack.
If a more sedentary style of exploring is in order, with a little bit of education thrown in subconsciously, don't be too quick to dismiss the Porthcurno Telegraph Museum. Check out the reviews, they speak for themselves.
For a bit of everything, head to Trethorne Leisure in Launceston. Bowling, farming, rock climbing, indoor-play. It's indoor entertainment in its entirety.