I don't have a daughter, but I think if you're not fatphobic, the sight of a thin actress isn't particularly difficult or complex to explain to a kid.
I imagine a hypothetical conversation with a daughter could go like this:
"Yeah, she does. Well, bodies come in all shapes and sizes."
"But she looks like she might be sick. Is she okay?"
"I don't know, she might be. That's her business, though."
Maybe this could lead into a discussion of systemic media sizeism -- bodies come in all shapes and sizes, but almost all the people we see on TV are thin instead of reflecting the vast diversity of the real world.
But, if you are fatphobic, then the sight of a very thin, potentially "unhealthily thin" actress becomes a crisis.
If you constantly frame thinness and weight loss as good, and fatness and weight gain as bad, and you emphasize the importance of "eating right" and "watching your weight," if you equate thinness with "health" and fatness with "illness," and you surround your daughter with good, aspirationally thin role models, then you're good and stumped if your daughter sees a woman who looks (to your judgment) "too thin" or "unhealthily thin."
Because now you have to walk back your messaging and emphasize "You're supposed to be thin, but not THAT thin. She has taken it Too Far. You must not take it Too Far like her." And you have to write dozens of Thinkpieces about the Challenges of Explaining Thin Actresses To Our Daughters, and isn't it really her fault for existing in public while thin and "unhealthy-looking" and complicating my Imparted Lesson that Thin Is Healthy?
If you're not fatphobic, there's nothing to explain.