Quick cover ups for churches in hot July weather

That will work
Knees and shoulders must be covered, not the entire leg

Some of the young gals on our tour stuffed a lightweight skirt plus headscarf into their daypacks for entering Mosques.

Notwithstanding what the bishops dresses and hats could let you believe, The Catholic Church isn't fashion conscious: As long as women cover their knees and their shoulders they are happy. The way you achieve it is not relevant to them, an "Hail Satan Lord of Chaos" T-shirt would be ok if it covers knees&shoulders.

Now, I am afraid You must solve the other half of the problem: Would a "sarong" type be acceptable for your daughter?

That's what my wife always used: a pareo wrapped around her waist. Make sure she has something covering the shoulders too if she's wearing something exposing them, a short sleeve t-shirt works

And the requirement applies to males also -- equal treatment.

Found this good option!
https://www.amazon.com/gp/product/B077BVDYKC/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Yes! And perhaps some photos or instructions (not from Mom, of course) on a nice conservative way to wear it!

Found this good option!

Yes. Perfect. Make sure she wears a garter on that leg. If the priest or church guard complains, she can tell him that she's a nun from the Order of the Garter.

My wife and I just got back from Italy and were amazed at how little the dress code was enforced. It seemed shoulders were prioritized mostly. But it was wildly inconsistent. I would still cover up, obviously. But it was just weird to see.