Here is a lovely August school holiday break to Croatia (Pula). Departing from London Stansted on 23/08 and returning on 30/08. This is based on a family of 3 (two adults and 1 child).
I think Croatia would be a lovely break, good value and as I've included hire car you also have the opportunity to explore the Croatian countryside.
The apartment is the Duga Uvalva (named after the nearby cove of the same name) which has three beds and will comfortably accommodate a family. It is right on the coast and there is a beach nearby. There is also restaurants, shops and a small beach bar.
Flights are with Jet2 and included hand luggage only £358.20 http://www.jet2.com
Car hire is with Argus Car Hire via a company called Fleet. I've looked at reviews and ratings and these seem really good. I can find nothing in the terms and conditions about mandatory extras, but this price only includes basic cover and if you have an accident the excess is around £2500 so you may want to take out additional cover. £25.29 https://www.arguscarhire.com//?c=gb
Book apartment £212. City tax is extra on arrival at the apartment but I'm sure the figure on booking.com is incorrect as children do not pay city tax! http://www.booking.com