Warm family beach early December
 

Somewhere within 3 hours of UK. 3 young children need their (late) Summer holiday.

Pretty tough to find within a 3 hour flight of London. If by "warm" you mean more than 65F/15 C you will not find warm weather in December anywhere in Europe or north Africa (like Morocco. You might try parts of the Middle East like Egypt or Oman, but that is more than 3 hours.

Take a look at weatherbase.com

I don't think you're going to find warm beach weather in Europe in December.

Canary Islands probably your only option if it has to be Europe. Not exactly sure on flight duration time though.

I don't think you will find any that are sure to have warm weather, within 3 hours of the UK. You might get lucky in the Canary Islands, but I wouldn't count on it. The nearest would be somewhere like Sharm El Sheik, or the Gulf States.

Can't think of anywhere within 3 hours. If you want great weather and water park then go to Dubai.You'll find plenty of Brits there.

If you can manage an extra hours flight the canary islands would be perfect. The weather in december is warm and resorts like Los Cristianos and Costa Adeje are really good for beach holidays with children. Puerto Rico in Gran canaria is also good but Fueta Ventura and Lanzarote can be very windy in december. We took our son to the canaries when he was little and we had a great time. Wherever you go I hope you enjoy it.
CM