All I can say is go for it, we will be doing a nine hour flight and five hour time change with a 7yr old 5yr old and 2 and a half month old, I always feel these things are as easy or as hard as you make it the calmer and more relaxed you are about it the calmer the children, as long as your prepared you'll be fine have a good time.
I would think at 9 and 5 they would be pretty good at entertaining themselves, especially the 9 year old. Gameboys are a god send on long flights. Books - coloring books for the 5 year old, regular books for the 9 year old. If the they are missing school and have homework to take along the flight is a perfect time to get a lot of it done.
When my DS was 3 we went to WDW. On the plane, we were told that we would have to circumvent a tropical depression in DC and take Ohio route. We were on the plane for 6 hours. My DS was fine. It was the adults that were complaining about the ride.
We bring a portable DVD, a book and games with us to chase the time.
have the kids flown that distance before? if no, prepare them by talking about what they can expect during the experience.
if they (like mine) are prone to ear infections, and your regular health insurance does not cover out of state non emergencies:get travel insurance. we ended up with both kids complaining of stuffy ears on the plane, which evolved into ear infections by day 2 of the trip. travel insurance paid all med bills with us only resp. for 50.00.
you are in the exact opposite situation from us-we travel from west coast to east coast disneyland in march is quite cool, can be downright cold so at least you won't be dealing with a time change in hot weather (the worst in my opinion!). we just try to go along with what our bodies are telling us-if its time to take a nap we do so, we don't try to get up at the moment the park opens if our bodies are telling us we need more sleep. don't try to adjust your sleep patterns too much, you will just have to readjust when you get back home.