Outed as Poly by an Eight-Year-Old

This afternoon, my eight-year-old daughter Sienna had a play date with one of her friends, at the friend's house. Sienna was picked up, and fifteen minutes later, I had this text exchange with the mother [I received permission to share, with names/identifiers redacted]:

Her: Sienna outed me on the drive over. "Are you a poly mama?" [My daughter] didn't miss a beat, and just started talking about another boyfriend I used to have, long before marriage.

Me: Wow. Whoopsies. Sienna has been wanting to converse with me about this [polyamory]...guess she's a bit too savvy for discretion. (Or not savvy enough.) Good thing you had that old boyfriend!

Her: I figured [my daughter] would hear it from Sienna first! I'm glad I was there to hear it. [My daughter] really didn't seem to see a relevant distinction between a boyfriend I had when I was single and one I have now.

Me: And why should there be? ;-)

Her: Exactly. And how nice (for both of them) to know that other kids' moms do this too.

Me: I do think that's a good thing. Definitely.

Her: My mom has been worried about the horrible shock should the girls learn of this disturbing arrangement. Ha!