I was secretly very pleased when my youngest child got a B in high school. It showed her that she did not need to be perfect, life still went on, her parents still loved her, and her friends were still her friends. We went out to get ice cream.
We did not worry about the impact it might have had on university admissions, and in retrospect it does not seem to have had any impact at all.
Letting a child experience a B sounds like a good plan to me. I do not think that it is anything to worry about, particularly if the B’s are rare.