How do you handle "you're not my real mom!"

When my 2 oldest boys said it, I was getting snacks. But I told them "us fake moms can't get snacks". After that, we had a convo about terminology. My 2 youngest never said it.

I read an article by an adoptee in which she described how she wished her mom had responded to comments like this (http://www.anneheffron.com/home/2017/2/9/an-adoptee-imagines-conversations-that-could-have-changed-her-life). Basically she said she would have liked some validation - a response like, "I am your real mom. But you have another real mom, too." And then open up the conversation to help the child understand that thinking about her birth family and even missing them is wholly acceptable and not disloyal to the family that's raising her. That was for a closed adoption, though.I have a friend who is an adoptive mom in a positive and healthy open adoption, and she responded with, "I am your real mom, and your birth mom is your real mom, too. Shall we call her and see if she disagrees with this parenting choice you're mad about right now?" She knew that her daughter's birth mom would back her up 100%.