Nothing wrong with acceleration/being too young in the class. Both of my DDs skipped to 1st. They never had problems and we will answer as a matter of factly. We really did not need to explain a lot.
This year, DD 10 is doing Algebra I and some of her older sister's friends are in the same class. They told my older daughter about her younger sister being smart and that's about it. It helps that it was not unprecedented.
Lisa Yee's book "Milicent Min-Girl Genius" is a good book about dealing with Muggles/nongifties
It is a fun book to read as well for your DC.
