I'm currently daydreaming looking at travel agents on the Internet and wondering how the bloody hell families even afford to go abroad to do or see anything at all.
Your Cornwall holiday might not be significantly cheaper than a week abroad. In Cornwall, accomodation is expensive, food and drink out is expensive, paid indoor activities when it rains are expensive, travel, parking etc are expensive.
OK if you go abroad you need passports, but that's only every 10 years. Accomodation will probably be cheaper and if you can be flexible with flights you can get some decent deals, or book a package - you can still do it this way and stay in an apartment or villa in a quiet area if you want to. In many countries, eating out, supermarket food and especially alcohol is cheaper.
Many paid activities are cheaper and it's much easier to park a car for free. We almost never have to pay to park the car in Spain, even in big cities or near the beach. And of course, because a lot of the time you have almost guaranteed good weather, you can spend a lot of time outside, whether at a pool, beach, playpark, walking/cycling, mountaineering or whatever and spend little or nothing.