when you start crunching numbers remember to take into consideration that for some things a child of 10 is priced the same as an adult (i thought when i looked at the dining plan it only allowed kids prices if they were under 9 or 10-which is a huge waste of money for our son, he prefers the kids meals and they are much less expensive to buy outright vs paying for an adult dining plan). i can't remember but i think once you hit 9 or 10 you can't get a child's park pass you have to get an adults. so don't budget based on 2 adults/2 kids it may end up being more like 3 adults/1 kid.
with the seaworld trip you will have to figure in either a one day rental car or some type of transportation.
the time of year you go will play a huge factor-you can get much better deals in the off seasons/non holidays (and you kind of have to play with the disney web site on this-because what you consider to be non peak might not mesh with their 'calendar'-(you think 'o.k. november will be crazy high prices thanksgiving week, but earlier in the month it will be cheaper.' in reality because of 'jersy week', 'soap opera' weekend and a few other factors (they get allot of austrailians there in nov/dec because it's their summer so they end up taking their big vacations) the rates jack up around vet's day (and it's insanely crowed). so if you have some ability to travel at different times of the year try all the dates-may save allot of $$$.
weekend rates are higher-so it can be a savings to go weekdays.
if you will only do one park per day don't get a park hopper ticket.
i know allot of people on these boards (myself included) book everything themselves and don't like package deals-but disney has had some realy good all inclusive packages lately that include your tickets, room, free transportation to and from the airport-and the wierd thing is sometimes while you pay a little more than you would by doing it all separate, if you need airfare you can end up saving by doing a package with the air included (we had this experience with a hawaii trip we're planning-i'm paying about $20 per day more for a room than if i were not doing a package BUT the package price including airfare is close to $1000.00 less than the best air prices i can find).
spending money? depends on your spending habits-figure prices for stuff are pretty much the same as non sale prices for the same stuff at the
disney store. if you are big beverage consumers figure it's pretty hard to find anything in the parks to drink for less than $1.75 (and thats for a small cup/container or a standard size can of soda/1/2 liter bottle of water).
extras? if you're not taking your own there's stroller rental, if the 3 year old is still a crib sleeper you might have crib rental (not sure how asm works it-i know the dvc rooms have free portacribs), if you are renting a car and don't bring your own carseats there's a fee for that, if you need a mini fridge in the room-it's like $10 a day, if you need any otc meds you'll pay insane prices for them on-site (i pack a lunch box with every otc known to man and put it in the suitcase 'just in case'), if your medical insurance is like mine and does'nt cover out of home state unless it's 'life threatening emergency' it's best to buy
travel insurance (both my kids ended up with ear infections after flying-i paid maybe $160 for a policy but it reimbursed me the $400 doctor's bill and the $80 in perscriptions-my pharmacy coverage does'nt work out of state either

), if you pack your suitcases when you leave home close to the 50 pound max and you plan on adding souvineers to bring home, unless you bring an extra suitcase plan on paying the airlines $50 for going over the weight max, we don't do bad on 'tips' (the ones at meals we figure into our food costs) but there are still the porters for you luggage at the airport, the hotel staff that carries your bags to your room...and if you rent a car and use it onsite the parking at the parks is not free/included in your ticket price, and don't forget whatever it costs for long term parking at your airport (we've found that depending on the length of stay-it's sometimes cheaper for us to use either a door to door shuttle service or book one of those airport motel deals where you stay the night before you leave, they let you leave your car in their lot and shuttle you back and forth to the airports), only other 'extra' i can think of is if your kids go to one of the 'kids clubs'-i think they run around $8 or $10 per hour for each child.
just as a suggestion (this from another mom with 2 kids)-if you end up with a return flight thats allot later than checkout and you don't want to do another run to the parks, but dread the idea of keeping the kids occupied at the airport-check out the orlando airport hyatt hotel's day rate packages. you can arrive at the airport, check the luggage, get your boarding passes and then go to the hotel (it's right next to the security area) and relax in a room for a few or several hours. we've found this to be a great deal-it's so much better having a bed to stretch out on while the kids watch 'sponge bob' vs. having them bored, whiney, tired with only those hard plastic chairs at the gates.