I just looked and came up with several options for you. Have you been only searching for a specific restaurant? You can leave the restaurant name blank and you will get results for all that have availability. I'm not sure what parks you will be in on which days, but here are some options that are out there right now, at pretty good places, and all at decent times (like between 4:30 and 7). I would gladly eat at any of these. Plus there are plenty more I'm not listing
Dec. 19: La Hacienda de San Angel, Raglan Road, Trails End, Tusker House, The Wave.
Dec. 20: Chefs de France, Captain's Grill
Dec. 21: Biergarten, Captain's Grill, Grand Floridian Cafe, The Wave
Dec. 22: same as 21st, plus Hollywood & Vine, San Angel
Dec. 23 - there's an Ohana right now for 6:20 - I'd snatch that one up fast!