I agree with the others that you should get there early. Try to be there a little BEFORE official opening time and head straight to Fantasyland.
I'm sure if you do a search for Fastpass, you'll find loads of info. It is available to everyone who has a park ticket, so please use it. In a nutshell, it allows you to bypass the regular standby time. Instead you get a ticket with a one hour window for your return time, to come back with little wait. Only select attractions at each park offer Fastpass.
I'm sure others have some "best practices" for using Fastpass and will add on to these. There are some things to keep in mind:
You can only have one Fastpass at a time. Check the bottom of the Fastpass to see when you can get your next one. It is usually about 5 minutes after the start time of your current one. We usually get our next Fastpass first and then go ride the one we have the ticket for.
Also, keep this in mind when looking at the start time of the window before you automatically get a Fastpass. I'll use Stitch's Great Escape as an example, although with your kids ages, I'm definitely NOT recommending this attraction. When we were down in Dec, the standby line for Stitch was 20 minutes. The Fastpass return time at about 11AM was not until 7:30 PM!! If we had gotten Fastpass for then, we would not have been able to get any others for most of the day.
If there's an attraction your kids really like, you can use Fastpass to get a "double dip". Get your Fastpass, wait in the Standby line, then ride again when your Fastpass comes up.
Check out the attractions and ask questions ahead of time so that you have an idea of which ones you would like to try. There are some great attractions in the other lands that you and the kids would probably enjoy also. Pick out a few "must see" rides and then have an idea of what else you'd like to do if there's time. Don't try to overplan though. Go with the flow, relax, and take your leads from the kids. Most of all, have a fun trip!
Mike
PS, Are you going with another adult? If so and there is a ride you really want to try but the kids can't go on, then you could do the child swap so you get a chance to experience it. Obviously, I wouldn't spend the day doing this (not really fair to the kids), but I would do it once for, say, Splash Mountain.