Thanksgiving is inclusive, isn't it? So whatever feels festive? I've been the only guest in a suit, and nobody cared. If you feel most celebratory in Manolos, maybe just bring protective paper booties to slide over them if a toddler shows up under the table. I'm sure the hosts will simply be glad for your company. And for a big piece of that pumpkin cornbread!
In my experience, presence of livestock is not necessarily an indicator of (in)formality. Not everyone takes a walk after dinner; if this family happens to think it's mandatory and you don't care to bring casual shoes, maybe it's a lovely opportunity for them to expand their range of practices along with family size.
I suppose that if I were in your shoes, I might ask DD if she had any pronounced preferences. My starter-ILs way back when tried very hard to get all of this right; I was not much help, because the familys were formality-compatible and other things mattered more. When I met keeper-MIL for the first time, we couldn't have been dressed more differently, but were both just happy for the chance to meet.
Life is short, there are llamas on the loose, and any occasion to come together in peace sounds like a very thankful holiday indeed.