If you have older kids that can manage a long flight then I'd really suggest Bali and there are some great flights with Garuda Indonesia which are a good airline in my opinion, I've flown with them long haul and short haul and never had problems. The flights include luggage and have a short stop en-route but it is still a long flight so I know this will put some people off but if you have kids a little older that don't mind travelling further then Bali is a great destination.
Flights leave on 20/08 but arrive into Bali on 21/08. You'll need to allow for transfers in your price but personally I'd look at taxi or shuttle options as Bali is so reasonably priced you can't go wrong.
I've picked a lovely hotel, and there are so many cheap options in Bali. It's the Koa D'Surfer hotel in Cangguu http://www.tripadvisor.co.uk/Hotel_Review-g311298-d8445384-Reviews-Koa_d_Surfer_Hotel-Canggu_Bali.html
The hotel has great reviews, a rooftop pool and is close to the beach. It is also surf themed which I thought was really uniquely designed and ideal for kids that enjoy surfing too.
Bali is really budget friendly for activities and to even hire a driver to explore the island. You'll certainly get more for your money too! *Based this trip on 2 adults and 2 children of 10 years old*
Book flights for 4 at Travel Trolley £1318.28
Book 13 Nights at the Koa D'Surfer with Ebookers using 14% off code which can be found in the voucher section of HUKD £398.69 the code is AFFBOX0217