Can I make a plea for your kids.
When i went my youngest was 10 and he found it OK.
Personally I think 5 is a big young to take round loads of big parks all day.
Make sure you dont just drag the two kids from park to park and ride to ride, all day, everyday.
They can get very tired, and very hot (and very bored)
If you have your wife's parents to look after the kids then great.
Maybe leave the kids (and parents) is a child friendly area (most parks have them) while you and you wife go for the thrill rides.
Islands of Adventure has a great area called Dr Zeuss landing which is just right for your kids.
They also have Toon Lagoon which is also great for young kids.
In fact I would have to say that Islands of Adventure is probably one of the best parks I have even been in.
It is magical from the moment you arrive to the moment you leave.
And the spiderman ride is amazing (a mixture of cartoon, special effects and thrills)
See here
http://orlando.citysearch.com/profile/11422026 /