You know, I would assume the same thing - that families and most people would be home, having big meals, opening presents, visiting, etc.
However, from what I have read from just a few folks who have been to DLR on Christmas day, I think it sounds as though it may start out kind of slow in the parks for the first few hours of the day, and then it gets super-insanely crowded from afternoon on. Then begins the week between Christmas and New Year's that is notorious for being either the most crowded or the second most crowded week of the entire year, because everyone is out of work and school, and/or armed with gift cards or gift money, ready to spend!
Also, I think a lot more folks are thinking of being at DLR on Christmas Day as a nice way to spend their holiday.
I would still go to DLR on Christmas Day if given the chance, but I would definitely expect very large crowds and have a plan in place of what to do.