They do realize. In early chapters when she first introduced to family, all of them (including the servants) think she's too tiny. That's why they give her so much cake, they're just silly. My poor baby Ruby suffered so much...
Yeah, I remember them thinking she was 6, not 8... but literally, she looks like a 3-year-old. But I wanted more of a reaction from them about why an 8-year-old is the same size as a 3-year-old. I wanted them to focus on that instead of giving her cake.
Maybe she looks even more tiny because the Northerner all buff and tall. She looks around 5-6 to me
I always wondered why she looked like a 3-year-old when she's actually 10. Finally, Khalid explained that she was in a constant state of using magic that affected her growth. I just wonder if her family realizes that she is way smaller for her age.