Last year and the year before we were lucky as DS1 was still 9 as we went twice within 10 months. This year we are going when they break up for the summer (as DS1 is in Yr6 and they are not allowed time off because of SATS etc and inset days) so we are having to pay full price for him.
However, if you buy any tickets on the day at the parks (we could probably get away with saying DS1 is only 9 as he is on the small side) I would do this as it will save you money.
Both of mine are on the short side (DS2 is 8), at the entrance of every ride they have the height charts and usually someone is stood there monitoring if they are unsure they will ask a child to be measured. The biggest and fastest rides have a height mimimum of 54" which both mine did not reach last year. I did Kraken alone.
Islands of Adventure is just fab. They will really, really, enjoy the 3d Spiderman ride (it is inside). The Hulk (I did this whilst they went on Spiderman (again) on my own) is another great one, but again 54"). Duelling Dragons - oh my god - this is two rollercoasters that are competing with each other and you are suspended ie. feet are dangling (so make sure your shoes are safe!). I went on my own again, then did a parent swap, so DH could go on. Basically, if DS's are not tall enough 54" one of you will go in a room near the front of the queue whilst the other is queuing, then when you come off, DH or yourself is sent straight to the front so you are not queuing twice if this makes sense.
The parks are vast so you need your walking legs on. Depending on what time of year you go (July for us will be really hot and humid) you need to drink plenty of water. There are drinking fountains around the parks and also lots and lots of stands selling cold drinks but obviously can be expensive.
Spending money depends on what you might like to buy really and to how many times you have to dip your hand into your purse during the day if DS's want snacks etc.
Food in the supermarket is cheap, very cheap. The first full day of us being there, we go to the supermarket and buy everything from toiletries to beer to big fresh fruit bowls to biscuits, to sun lotion.
A couple of times last year when we vegged round the pool, I bought salad things and huge tiger prawns and we ate in, although DH tells me we don't go away for me to do the cooking thing, but it did not take any effort.
We like to go the Kobe Steak House (Japanese) and you sit round a huge table with other people and in the middle it is like a big hotplate. You get your own chef for the evening and he entertains the kids. Think last year it cost us around $120 dollars for the meal and the drinks but we met a friend so that was for three adults and two children (who do not eat childrens portions).
Personally, I would not eat meals in the parks, you usually find that they are overpriced and very busy. I would prefer to take snacks and fruit and then maybe have an icecream in the afternoon.
A lot of the restaurants do kids eat free with an adult main. We like to go to TGI's, Kobe, Houlihans, there is another place on I Drive with huge stonking trucks outside - cannot remember the name but you will see it, and of course Hard Rock Cafe is brill - this is on City Walk which leads into Islands of Adventure and Universal.
We normally take around $800 dollars in cash, put the supermarket shopping on the credit card, then just top up the cash when needed. Would probably think you will go through in sterling about £1200.
When you go to Seaworld, there are 2 shamu shows a day and they are fit to bursting with people. Say the show starts at 11.00, you need to be there for 10.00 to get your seat. The soak zone really is the soak zone even if you are sat on the last bench of the zone - believe me!