K.B.
If the teacher assigned to the class is experienced or capable of handling a split class, then I think it would be fine. I was in a split 2-3 class in 2nd grade and a split 3-4-5-6 class in 3rd and 4th grade. I didn't have any issues and I actually kind of liked it because there were opportunities to advance beyond my grade level.
My kids are still in pre-school, but the local elementary school does split level classes for K-1 and 2-3. The students get to stay with the same teacher for 2 years so it provides some consistency. As long as both grades in the class are appropriately challenged, I don't see it being an issue. I think it will be a plus more than a minus.
I would prefer a smaller split-grade class over a large single grade class. I would be annoyed that they didn't know this ahead of time. It's true that people move around and change their minds, but you think that they would know before the first day of school and worked it out before school started.