My daughter announced to me last month that a little boy in her preschool class had asked her to marry him. And that she'd accepted.

I told her it was okay for her to have boys as friends, but that she does not have a boyfriend. She said, "right, because I'm too little." But, she was still flattered, I think, that a boy thought she was cute or whatever.
I let it go but made the point.
AND, when I was an elementary schooler, it was common for every boy and girl to say they had this boyfriend or that girlfriend. If it's gonna be in the culture, it's gonna be in the kids' culture. We have to frame the context for the words that get used, and we can't prevent those words from reaching our kids' vocabularies. (We can bring consequences, but we can't be full barriers.)
Human government bears the same relation to hell as the church bears to heaven. (David Lipscomb, On Civil Government, 72).