Or you could say, " Junior College XYZ has genetics course 101 opening soon, I could get you a brochure". Truly though, a blank stare and shake of the head is more appropriate. Your kids will know that looking different is a good thing if you teach them.
Our niece has twin boys. She is of Swedish descent, her husband is Hispanic. One boy is white, blond, and likes everything in place. His brother is dark, black hair, and wants to race cars. If you see the boys interact, you know they are twins.
My ex was of French/Irish heritage. With curly black hair (he once had me apply a hair relaxer-lol) and olive skin he would often have people speaking Spanish to him because they thought he was Mexican.
That old saying "you can't judge a book by its cover" is true. You can certainly judge some people by their stupidity.
Children that are loved can deal with the bad stuff.
Best Wishes,
Margie