The reason I asked where on the island you'd be based is because it may be miles from a beach and if it's beside one, is it a sandy beach or a volcanic stoney one, has the accomodation got a/c, has it a good sized pool or a kids one, what is there for kids to do, has the hotel got a good rating, etc, etc? If you're a distance from everything in Fuerteventura you could be in trouble with children and the places i've been to are not really renowned for catering for kids. Corralejo and Costa De Fuste have beaches but not much more and 2 weeks eating and doing the same thing every day will test your patience. As for extra spending? Of course you'll spend more than what the holiday cost, how much will depend on the kids liking of what they eat and drink. The drinks won't be what you usually get, Coke, ice cream, savories, etc, for example will be a substitute, same with all other stuff and i'd say another grand at least will be needed which isn't much more than a €10 each per day if you're lucky. Also it takes around 4 hours and 10 mins to fly there! Then again we're all different so good luck, but it wouldn't be my choice.