In our family were DH and I, two kids ages 10 and 8. Roughly, we spent:
$250 on excursions
$80 spa
$120 photos
$300 gratuities
$250 onboard souveniers, occasional drink, arcade, bingo
and off the ship, I would say we spent another $100 dollars on the islands (not Castaway Cay)
I think buying Disney Dollars over the period leading up to the trip is a fabulous idea! I will do that for our next trip.
The photos will add up quickly - we did get some EXCELLENT family photos on the cruise.
Another idea - I have started using
www.mypoints.com (more info is on the Disney Rewards board on this website) - my plan is to redeem those points to (1) purchase items for family use on the cruise and (2) purchase phone cards to give as appreciation tips for outstanding service on board. One thing I am going to check is if I can purchase a Disney Gift Card through My Points and then purchase Disney Dollars with the Gift Card - is that possible?
And one money saving idea for you - if you are thinking of renting floats at Castaway Cay - consider going to your local Wal-Mart and buying cheap, bright color (not blue) floats - take them along, blow them up at Castaway Cay and then throw them out if you don't want to bring them home. The advantages are (1) you bring a $2 float instead of renting, (2) it is easy to identify and locate your own kids by their color float in the sea of Disney floats and (3) if you leave the float you have some room in your suitcase for that t-shirt you are going to buy at Castaway Cay. Of course, the idea of throwing away the float is so typical of the American consumer attitude (read here a twinge of economic and environmental guilt), but....