Welcome to the DIS! You've already gotten lots of good advice and I just want to second some and add a couple more. I also recommend on-site but I'm not familiar with the place you're staying. If dgaston says it's good and close then it should be okay.
We went last year at the time you're going and the afternoon rest/cooldown breaks are a must, plus it rained a lot and sometimes we got tired of slogging around. It's just really nice to make that quick trip to your room to regroup and on site is quicker. Disney does have some deals on rooms and I think your family would enjoy the Fort Wilderness Campground cabins too. Fun and conveniant. Check out
www.mousesavers.com for tons of money saving advice.
I used to have relatives that lived in Orlando and used to stay with them. We would get tired and leave the parks always saying that we would go back that night, but we never did. The hassles of driving through the ungodly traffic all the way back there just seemed too much. So actually we weren't getting our moneys worth out of our passes. But that was just us.
Also, I would skip KSC this trip too. At the time of year you're going, you'll do well to get in WDW and Universal. If your girls arn't into rollercoaster thrill rides, I'd even skip Universal as that's what it mostly is...IMO. If they're into the big scary rides go for it.
Pick up the Unoffical Guide to Walt Disney World. It's a great book that will give you tons of information and touring plans. But, don't let it scare or overwhelm you. You've got plenty of time to plan. This is supposed to be fun!
Do check out Deb Wills website that someone else gave you, it has some stuff that this site doesn't have, but we've got stuff they don't. Both are great.
One thing that really helps is to study the maps of the parks before you go. That way you'll have a general idea of what attraction is in what park and the "land" that it's in. You're not wasting time getting lost. Which brings up another reason to stay onsite and use WDW transportation is you don't waste time getting lost!
I'll second getting to the parks before opening time. Most vacationers like to sleep in and arrive around 10am so you can really cover a lot of territory by arriving at park opening. By the time the hoards arrive, you've got the biggies done and just crusing along.
If you're not staying onsite then learn when the parks are open early to WDW resort guests and stay away from that park that day as it's twice as crowded.
Anyway, I'm getting too long winded here, but just buy the Unoffical Guide and it will tell you lots and ask questions here.
Another thing, if it gets too hot & crowded and the lines are too long and the girls are whining and you're getting frustrated....take a break and go to the room. You can't force fun.
