The article mentioned that it’s possible - likely the kids contracted covid while outside of school. So it’s like saying if you only put up your umbrella to avoid getting rained on on mondays but the rest of the week you don’t have / use an umbrella, then you say “people still got wet so that means umbrellas don’t work”.. so if there was a way to say “did the kids catch COVID while at school” this data would be interesting but without controlling for other possible sources of infection this is not super useful data.. like if the data showed masks help, I would say this data also would be suspicious for the same reasons..

Do they specify what kind of masks ?

It seems like masks, plus ventilation plus air filtering is ideal.

What about their parents?

Correlation does not imply causation.

The result of the experiment in the article was that there was no correlation.