Ubiq makes a really good point. Movies like Tangled and How To Train Your Dragon didn't have anything close this competition (well, Tangled had Harry Potter part 1, but that's about it and even that wasn't one of the strongest movie of the franchise unlike what's being said about Skyfall and Twilight)
The Polar Express was also in the same boat. It consistently did well sure, but during it's debut, it's biggest competition was The Incredibles funny enough. National Treasure was the only other big hit, and Ocean's Twelves came out roughly three-four weeks after the Polar Express' debut.
As for family movies aimed at children though, Wreck it Ralph has been cooling down (I still want to see it), and I can see a fair chunk of families still watching this for another week or two. As for Monsters Inc., I don't doubt it will take away some viewers, but if it's like Finding Nemo 3D it won't have a substantial audience, though given RotG's current status a showing like that could be a big blow.
Here's another fun tidbit I found, box Christmas movies at box office mojo. Now admittedly, a lot of these movies are old and the more recent Christmas films suck, but it does show that holiday films don't automatically equal success.
That said, I'm not going to just make excuses. The story isn't amazing, and the reviews have been mixed. If it was a stronger movie I doubt this would be an issue. But we'll see in a few weeks if it can consistently keep up its box office revenue (it'll be lucky if it can maintain weekly revenue in the range of $20-30 million for a bit, but I'm not sure that's likely).