Well, you don't provide enough information to help you completely, but here's some preliminary information for you:
First, you need to examine a few things: 1.) How long are you staying? 2.) What do you plan on doing? 3.) How old are your kids?
In addition to the 4 major theme parks (Magic Kingdom, Epcot, Disney Studios, Animal Kingdom), Disney also has an indoor interactive theme park (Disney Quest), two water parks (Typhoon Lagoon, Blizzard Beach), a nightclub district (Pleasure Island), and a sports complex (Wide World of Sports).
Disney has five types of admission: single day tickets, park hopper passes, park hopper plus passes, ultimate park hopper passes, and annual passes.
The single day tickets provide exactly that. One day of admission to any of the four major theme parks. You can't "park hop". Park Hopping means visiting different theme parks on the same day.
Park hopper passes provide you with either 4 or 5 days of admission to the 4 major theme parks. These passes allow to visit multiple theme parks on the same day. Unused days will never expire.
Park hopper plus passes provide you with 5, 6, or 7 days of admission to the 4 major theme parks. They also come with a certain number of "options". Options on the park hopper plus will get you into either a water park, Pleasure Island, or Wide World of Sports. Each of those will cost you 1 option from your pass. 5 day passes come with 2 options. 6 day passes come with 3 options, and 7 day passes come with 4 options. Unused days will never expire.
Ultimate park hopper passes are for Disney resort guests only. You can't get these if you're not staying at a Disney resort. They are good from the moment you check in, until midnight of the day you check out. They will get you into ANYTHING on Disney property as many times as you want. They are also the ONLY type of pass that includes admission to Disney Quest.
Annual passes come in two different types: An annual pass, and a premium annual pass. The annual pass gives you admission to the 4 major theme parks for one year. The premium annual pass is like a year long ultimate park hopper pass. It gets you into everything on Disney property for a full year.
Unless you're staying for 8 days or more, don't even consider the annual passes as an option. They're really pricey. An annual pass is about $390 after taxes, and a premium annual pass is about $520 after taxes. (Those are adult prices.....kids are somewhat cheaper)
If your kids are 2 and under, they get in for free. No admission needed. If your kids are between 3 and 9, you will have to purchase them a child's admission. If your kids are 10 and up, Disney considers them an adult for admission purposes, and they'll need an adult pass.
From there, the type of ticket you would need to buy is really dependant on how long you're going to stay. If you've got a shorter stay, I wouldn't reccomend the Ultimate Hoppers. There's just too much to do, and you'll be paying for features that you probably won't use.
If you give more information (those three questions I first asked at the beginning), I'd be more than happy to help further.