I'm taking the family for Easter, and we're budgeting $200 per day.
we've already bought the theme park tickets though - for 3 adults (our 10yr old's an adult at Disney !) and 1 child we paid �1250(that's for '14 day' disney passes, and 5 visits to Universal's parks). (you can also get good deals at the Theme Park ticket booths on I-Drive)
Food-wise, most places do a good, cheap buffet breakfast.
Lots of restaurants offer cut-price kids meals, and some will offer free kids meals with adults paying.
A credit card is an easy option - I've got a Post Office one, which doesn't have any foreign exchange fees, and should be just as convenient as cash.
Have you got any books ? I bought the Brits guide to orlando by simon Veness, and it's full of very useful info.