Look into it for sure, but go through the ranks. Reporting it to DHS would NOT be my first step.
First thing Monday, before putting your child back in class, either make an appointment to speak with the teacher (PLEASE do NOT confront her in front of other students or parents, or right before they'll be arriving). Or you could just go into talk to your aunt and let your aunt call this teacher down to her office to conference with the two of you while a sub watches her class or whatever.
If she really said this, it sounds totally inappropriate, but sometimes children misunderstand or remember things incorrectly. Another child could have actually said it, or the teacher could have been repeating back what another child said, etc. There are always many more possibilities than the obvious, so it pays to ask the teacher (and maybe the other child, with the parent's or your aunt's permission) for their version of what happened, and then take it from there. Give the teacher a chance to tell her side of the story, and then listen to your instincts about what to do from there. If you feel your aunt isn't taking it seriously enough, then consider pulling your child and/or making a report to DHS or against her licensing bureau, etc.
Also, sometimes adults use sarcasm or humor that children are too young to understand, and so they take it literally. In those cases, the adult needs to learn to be more appropriate, of course, but I have heard some people use phrases like that on their own children even when they had no intentions of hitting them.
Definitely take it seriously, but don't let emotions cloud your ability to think clearly.
Good luck!