I have no clue about US schools (yet)

I do know that 20 something years ago I was taught about Romans, Vikings and Picts in elementary school, and diddley-squat about modern history until high school.

A couple of possibilities........

Vikings and Romans are not boring, like any non-war realted modern history is. It's easy, you get to draw cool ships and there is pretty much no chance of any ethnic minority being offended.
Would you really want to get into modern politics or the details of WW2 with kids under 12? What would be the point?

If your kids are learning about Injuns, I think
a) wish I had (in additions to vikings)
b) believe it or not that is part of American history too
c) they are LEARNING - plenty of time for the boring stuff later.