As others have said, go to
Mousesavers.com to look for ticket deals. Anyone they recommend will be reputable. It's also a good resource for other Disney saving tips.
With kids that age, more days is better. If you're there for 10 days, you might as well get the 10 day hoppers (there's less than $10 difference between the 7 and 10 day tickets).
You can also save some money by getting the non-hopper tickets if you don't plan on visiting more than one park in a given day.
Keep in mind that Disney World is huge (twice the size of Manhattan island), and it takes a minimum of a half hour to get from one park to another. To get from Animal Kingdom to the Magic Kingdom would probably be close to an hour.
The good thing about having a hopper pass is that you can do one park in the morning, go back to the hotel for a couple of hours in the afternoon, then head to another park in the evening, but with kids that are that young, those really long days might burn them out.