Just get used to it because there is more coming!
My son was yelled at in the grocery store for putting gum with our purchases. I went to say that it was okay when she interrupted "It is NOT okay. He was trying to get you to buy his gum!" and I sheepishly mention "Well since he's my son..."
Once I tried to pick up my dd from school and was told that it was nice of me but they had to call the mother first.
Both my dh and I are white and Jewish. Yes, it still happens with the same background. I'm not pure Jewish (oh those questions!) so people assume I'm not (converted, or whatever). He's much darker than I am and the children look like dh.
What happens to us is that we're in France and I have an American accent (well, it is but they think I'm British). The children, born here, obviously have no accent. For some reason, they don't "get" that a foreign mother can have children who don't.
Sometimes it works in my favor. Dh is from a small village. i walked in to a bakery in his home town with our ds and the ladies esclaimed "Oh you're XX's wife! Oh and look at his little version..." Well I guess we don't have to introduce ourselves!
I think it's easier when you have more than one child and they look like each other. Then people would either you internationally adopted from the same country or, more likely, the kids look like the dad, who is "something".
Once a German woman boarded my flight with her three beautiful daughters. I'm a former Flight Attendant. One girl was dark and Indian looking, the other looked like she could be from South America, Hispanic-looking and the third was blond with blue eyes. I had to ask. "What's the story?" I simply asked her (giving her leeway to answer in the vague). Her husband was half Indian while she was typically blond German. Funny how the genes got passed down.
The mom got all upset, not at me but she said "People think they have different fathers!" Okay, I can see how they could come to that conclusion but who cares what other people think, I told her. I didn't want to preach to her but I wanted to say "So what if they do. It's no sin to have different children with different fathers" but she obviously didn't need to hear that!
I have two girlfriends adopting internationally children of different race who both already have "bio" children. I hope neither get any "dumb" comments.