Check out the abstract for this paper. Schoolkids six-years old and over were forced to wear masks, while kids five and under were not. Did the masks work? Excerpt:
Findings: SARS-CoV-2 incidence was significantly lower in preschool than in primary education, and an age-dependent trend was observed. Children aged 3 and 4 showed lower outcomes for all the analysed epidemiological variables, while children aged 11 had the higher values. Six-year-old children showed higher incidence than 5 year-olds …
Interpretation: FCM mandates in schools were not associated with lower SARS-CoV-2 incidence or transmission, suggesting that this intervention was not effective. Instead, age-dependency was the most important factor in explaining the transmission risk for children attending school.
Source: Social Science Research Network, 2022, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4046809.
The problem is that association does not prove causation.
There may be differences between the younger and older children other than their use of masks that could affect the incidence and transmission of COVID.
It is not clear that the study could have statistically controlled for all such factors.
That is why randomized controlled trials, truly experimental studies, are considered much more reliable indicators of causation