Well, I don't know what anyone means by Minnesota slang. I've lived her for 22 years, since I was eight, and the only thing I've noticed is that they call casseroles "hotdishes."
However, I would ABSOLUTELY correct this right away, because speech habits can be VERY hard to break if you let them continue for a length of time. It becomes an accent, almost. And I agree that pronouncing it that way does sound "ignorant." I don't like to label, so I won't, but I know exactly what it sounds like to me. Where in the world do you suppose she would've picked up such a lazy way of speaking? Weird, especially since you homeschool.
I think you should just gently correct her whenever she says it, during "school" hours or not. My own five year old had just a few genuine speech issues to address, and in just a few months of consistently addressing them, she has outgrown them. It can be done.
With this one, especially, I would nip it in the bud or before you know, she will be saying, "Ma, can I Aks you something?" Hmm.
Good luck! I think you are on the right track, just stick with it!