I'd think about if this is really the sort of critique you want to give another writer on a creative piece of work, though.
"They had a look of worry on their faces, but I knew they weren't."
Not sure if the marks are to show a character is talking, or you've added them in to highlight the quoted work, but it's either speech or it's first person (so thought) and most people do not speak or think in perfect grammar. In fact, changing it to "Despite looking worried, I could see that their faces were not genuine" would be criminal if this is speech and wrong for first person unless the character was A) extremely eloquent and B) really thinking about what they were thinking... maybe musing after an event. Nobody would "think" that way while the people were in front of them. That's just a rewritten sentence, and it sticks out in first person a mile away.
Since the meaning of the sentence is abundantly clear (and shows the voice of the character talking / thinking) it's absolutely fine.
So I'm not sure why you'd crit the grammar of speech or thought unless it was clearly a "rule" the writer just didn't understand, like repeatedly getting confused about 's or capitalising / not capitalising Father. But honestly an editor / proofreader / program like ProWritingAid is going to pick that up anyway.
If you want to give them critique that might be more useful, I'd point out that they could enhance this by putting the look of worry into the readers head. Control them. Make them see exactly what you the writer want them to see.
And they could enhance it again by removing the "I knew", which is just filter and (unless it's an intentional / stylistic choice) keeps the reader a step away from the story.
Sarah bit her lip. Joey shifted from foot to foot. They were good at looking worried when they had to be.