Tickets are expensive but worth every penny. Especially if you have 4 and 5 year olds.
I just went in mid October and it wasn't bad at all. Micky's not so Scary Haloween party was so much fun - but pricey! They say the best time to go is the first week in December. It's the slowest time - but you have to pull your kids out of school.
The best parks for 4 and 5 year olds are Magic Kingdom (of course) and IMO, Disney's Hollywood Studios. DHS has a bunch of live shows that my 5 YO LOVED. And so did we. The mid-day parade is awesome and the Buzz Lightyear (get Fastpass early) is fabulous! The Indiana Jones show is great, too! It's all good at DHS. And you can do the whole park in a day. Magic Kingdom really takes two days. Skip Epcot since the kids are young. Animal Kingdom is great too. But it's a lot of walking...
Since you will have a car and are staying off property, you can get great prices and specials on food. Try to only eat one meal on property if you can. Since you have 4/5 year olds, you will need to nap at some point. Grab food then.
Also, grab a copy of the Unofficial Guide to Disney with Kids. It's priceless!
Have fun... It's worth the splurge!