Congratulations on your upcoming FIRST trip to Walt Disney World. I truly hope your family has a magical vacation.
I would pack layers although the weather should be pleasantly warm without the really high temperatures and humidity that are seen during the summer months.
One source that I highly recommend is using
www.wunderground.com. By going to Orlando and then clicking on historical data, you can see what this exact week has been like in previous years.
For 2007, highs were 71-90 and lows were 65-75. It's quite a range. I pack layers, then I watch the forecast. As I get closer to our departure date, I can then adjust the contents of our luggage according to the latest forecast. Sometimes I've needed to add more pants and other times more shorts. There was one vacation in January where the forecast predicted cooler temperatures but we ended up wearing more shorts in the unanticipated "heat wave".
I'll take a pair of jeans, pair of shorts, matching shirt and roll them together before placing them in my suitcase. If the forecast is then for hot days, I'll remove pants from a few outfits.