Hi Colleen,
I love your Disney spirit!
MagicBands are delivered to your home approximately one month prior to your reservation date. This is all automated and there's not really any way to push the system to deliver earlier. So unfortunately, I don't think your plan for MagicBands under the tree will work.
There are, however, lots of other wonderful Disney-related items you could wrap up to convey the same sense of fun and excitement. Over at DisneyStore.com, there is an entire section of theme park related items that you can have delivered to your home at any time. A classic example would be
Mickey Ears embroidered with your family members names', but you could also do a WDW themed ornament, trading pins, costumes, plush character toys, character autograph books, or countless other items.
Your local party supply store probably has an array of Disney theme balloons for sale. You could inflate several with helium and wrap them in a box, so that when the box is opened, the balloons fly out. You may find other ideas in this article from independent fan site Touring Plans on How to Surprise Your Kids with a Trip to Disney World: http://blog.touringplans.com/2012/12/05/how-to-surprise-your-kids-with-a-trip-to-walt-disney-world/
Good luck with your planning. Please let us know what else we can do to help.