You have you be really careful interpreting reports of papers.
If you just read news.harvard.edu/gazette/story/2020/08/looking-at-children-as-the-silent-spreaders-of-sars-cov-2/ the headline is:
In a study of 192 children ages 0-22, 49 children tested positive for SARS-CoV-2, and an additional 18 children had late-onset, COVID-19-related illness. The infected children were shown to have a significantly higher level of virus in their airways than hospitalized adults in ICUs for COVID-19 treatment, according to Harvard-affiliated Massachusetts General Hospital (MGH) and Mass General Hospital for Children (MGHfC).
- This is a relatively small number of "children" where they define a child to be up to 22 years old.
- It says nothing at all about how likely they are to infect adults.
If you look at onlinelibrary.wiley.com/doi/full/10.1111/apa.15371 for example, it says:
"Household transmission studies showed that children were rarely the index case and case studies suggested that children with COVID‐19 seldom caused outbreaks. However, it is highly likely that children can transmit the SARS‐COV‐2 virus, which causes COVID‐19, and even asymptomatic children can have viral loads."
And then we can look around the world (or even just Scotland) where schools have been opened and again, we see no clear picture that this has causes large outbreaks.
The punchline is that no one knows for sure but England/Wales are not the first countries in the world to open their schools so we can be reassured by the experiences of other countries. We also have to, as ever, weigh up the immense risk to an entire generation of children of not opening schools.