I would definately suggest mid September. If this is your first Disney trip EVERYTHING will be new and exciting. I wouldn't worry about the special add-on parties etc. You'll be busy enough!
1st recommendation, buy the Unofficial Guide. It has tons of info. You don't have to "go commando", but it gives you a good idea of how to hit the parks to get the most out of your trip.
You didn't mention how long you planned on being there. If you have the time, plan for:
2 days at Magic Kingdom
1 day at EPCOT
1 day at Studios
1 day at Animal Kingdom
1 day at Typhoon Lagoon (optional)
BB is a toughie, it really depends on your kids
Set aside time for Downtown Disney. If your kids like
Lego, go to the Lego store there. Our kids loved Cirque Du Soliel, but it depends...
Be sure to plan in resort/pool time. Kids in the age ranges you mentioned remember the pool and resorts as much or more than the parks...
There is a ton of info on this board. You'll have a great time! Check the links in my sig block. They should help. Feel free to PM or email me as well. I always have opinions
