Unfortunately, I can't access the full text of the study directly, but I would argue it would be more accurate to say it impacted (past tense) 1 million children, assuming this paragraph from the news story is correct:

> Results of the analysis, published in the journal JAMA Pediatrics, showed approximately 1.01 million children, or 1.4%, are believed to have ever experienced long COVID as of 2023 and about 293,000, or 0.4%, were experiencing the condition when the survey was being conducted.

There are certainly reasons to think it might be under-reported, so perhaps that should go in the headline (or subheadline) as well. But I don't think the headline should suggest this particular study estimated 1 million children were experiencing it at the time.