Go for it! We've spent the actual Christmas in WDW several times..luckily my parents and sister have gone with so that eases my guilt about the kids/grandparents thing. Inlaws have never gone, but are always ok with it- They say TAKE ADVANTAGE OF IT if we can! We just do their x-mas before or after. Actually it makes Christmas for us and them. Instead of cramming 3-4 family Christmas gatherings in to two days. We can actually enjoy the holiday and it makes it last a couple weeks since we do the family stuff before or after. Plus, my kids see both sets of grandparents several times each week and usually talk to them everyday. So getting together on Christmas for us is like a normal day with the added presents and turkey. I guess if you don't see your families that much, I could understand that it's more of a special gathering, as it should be!
As for presents, Santa always brings some special Disney gifts and puts them under our "tree." One year it was a small artifical tree (ordered though Disney florist- very pretty and easy, but kind of pricey- plus we had to ship it home). Another year my then 3 year old and her grandma colored and cut one out and then we taped it to the wall (asked the shop at the resort for large paper...the stuff they wrap breakables in). They also used the paper and made a fireplace which we taped on the door so Santa could get in. The people at the resort shop also gave us some old postcards and little "giveaways" (like the mickey shaped plastic balloon weights) to use as ornaments and they gave us silver paper for a star too. They were so awesome- they actually kind of got into it too! I also bought some Mickey shaped ornaments and taped those on our "tree." It was a really fun way to make Christmas OURS and make it special. After all, we had a tree at home already...
We also brought Disney stockings with us to hang (cheaper from a Target, Menards, etc., or get them at
Disney Store/DisneyShopping.com) These are EASY to take with since they're flat!!! I just waited and picked up stuffers at Downtown Disney and the Orlando
Walmart/Target (being Orlando, they have great selections of Mickey products- especially stocking stuffer size- for pretty cheap).
As for other presents, when we get home there's always Santa's big delivery waiting for us (yes my kids are incredibly spoiled). Either have someone come and put out the stuff an hour or so before you plan to arrive home or if you arrive home late in the night like we always do, you'll be carrying the kids to bed anyways so just do it then.
Whatever you decide to do- have fun! And good luck with the family, hopefully it'll work out that they can go with!