Consider your family's personality:
If you're more fast-paced want to be out all day at the parks, constantly busy and surrounded by people, stay up late then maybe 4-5 days would be your speed.
If you want to relax, take lots of breaks, and enjoy the resorts a 10+ day stay would be wonderful.
With the exception of our most recent trip (one week) we have always gone for 11 nights to two weeks. Especially when our kids were little it was really nice to get up early and hit the parks for 3-4 hours, then go back to the room to relax (get out of the heat and away from the crowds, sometimes nap) until maybe 5pm and then go back out for a few hours. We always get park hoppers so we can be flexible and with 10 park days times two parks each day we never felt pressured to do a lot all at once because we knew we could come back another time. If we were only there for a few days, I just don't think we would be willing to leave the park for a break because we wouldn't have enough time to get to all our "must-see" attractions. Personally, being a park from rope drop to close and trying to fit in everything to "get my money's worth" (especially with young children) sounds like an over-stimulation nightmare, but some people thrive on the excitement.