I'm in the same position as Heifer - was adopted as a baby, (adopted) mum has passed away
I constantly came up against this as a child. "Your mum isn't your real mum," "your real mum didn't want you," and the real doozer, "you're secondhand" (gotta love what kids say, huh?! -this was said by a 9-year-old!!).
My mum taught me to say that "my parents CHOSE me whereas yours just got stuck with you!" which is, admittedly, not very kind, but it usually shuts people up. If the child saying the "not your real mum" stuff is under the age of, say, 9 or 10, they probably aren't being malicious, and are just curious about a situation that they haven't encountered before. (I say "probably" as there's bound to be someone out there with different experience of this!). And I also used to say, "My mum is my real mum, but I didn't come out of her tummy" or similar.
Incidentally OP, I was six when my parents explained the adoption to me in detail. I understood exactly what they were talking about, and had no problems with any of it at all. Thought it might be helpful to know this as I think you said your DD is six too ..?? Good luck with this