I agree with you that it is possible for adults to engage in age inappropriate relationships but that doesn't make them not adults suddenly.

It's age inappropriate for a 70 year old to be dating a 30 year old, but that doesn't make the 30 year old a child.

Besides which, we're not talking about someone having sex with this woman, we're talking about whether it is appropriate to refer to her as a child. I would argue that she is not a child and therefore it is infantalising her to the point of disrespectful rudeness to refer to her as a child.

Moreover she was in a supervisory position. You don't put a child in a role that has supervisory responsibilities.