OP, what would your alternative seating have been? I have a 2.5 year old and I absolutely need to sit with her to eat. Do you think another adult should have sat with her?
Given that the tables were a foot apart, and I'm sure you could have mingled in the room, I don't think the hosts were rude. If your child was clearly old enough to feed his/herself, that's another story.
You say your husband's family doesn't get to see each other a lot, I don't see the point in putting your husband at the kids table too. I guess I just don't. If you're feeding your little one or assisting anyway, you're probably tied up with that. It's easy to read this as rude but perhaps look at it as practical.
If your kids table was in another room, as my IL's have every Xmas Eve, then I'd say 100% rude if they seated you there. I voluntarily sit at the kids table with my little ones and am usually the only adult there. Even better, they close the door between rooms to keep the noise down. But that's sort of what I figure is what happens when you have small children, until they can tend to themselves. Two table heights next to each other is hardly exclusionary.
Well, one possibility would have been for hosting SSIL to sit at the kids' table instead of me. 2 of the kids were hers (and frankly her hyperactive 9-year-old required a lot more "handling" than my DD, so she was up from her seat dealing with him anyway), her in-law status is the same as mine, and she is the one who chose the arrangement of chairs and tables. If she assumed that DD would need to sit next to a parent (which I agree is not unreasonable), she could have seated DD at the main table with me and DH and seated her own mother and sister, who were both single guests and the only attendees from her side of the family, at the small table with her kids. Or she could have put 2 adult couples at the small table and all the kids at the big table. Or put me, DH, DD, and DH's single stepbrother at the small table. There are a lot of possibilities that wouldn't have involved isolating one adult.
Not that it matters, but aside from feeling excluded, I was also uncomfortable because I am not a kid person (despite having one). I am not good at making conversation with other people's children.
It was not possible to talk with the other adults from where I was sitting. The closest people had their backs to me and their heads were several feet above mine.