Iceland.
Easyjet flights get released about third week September for May half termmand will probably be £20 - £40 each way each so flight budget with seat reservations around £300 - £400 if you just travel with a generous hand luggage bag each.
Accomodation in 3/ 4 bedroom house with essential Icelandic piped hot water tub around £700.
Car hire for a nice off road capable vehicle £650 (yes its expensive).
Fuel £100
Food not much different to UK if you self cater and don't drink much alcohol but you can spen a fortune if you choose.
Days out...
Swimming in thermal swimming pool with slides about £12 for the family
Geyser (geysirs as there are several active) just fuel as free parking
Gulfoss, very impressive split waterfall at end of glacier. Free parking, just fuel.
Thermal gardens and cooking eggs in a stream about £10 including eggs.
Whale watching including oilskins lifejackets etc about £100,for the family ( children are typically free).
We went when our youngest was three, went back when she was four and plan to go back next year when shes six. Its a very child friendly culture and English is very widely spoken and written.