An important fact to note about that study is that whilst infections in under 10s were low (6.7%), in over 10s they were higher than in the general population (13.7% for over 10s, vs 13.3% for the general population).
That would mean children in years 5-13 are at as much risk as adults of carrying and spreading CV19. Reception and nursery children are at lower risk - about half the risk - but will still carry and transmit it.
It must also be considered that in Iceland children don't start school until they are 6 - while in the UK they attend from 4, and sometimes younger. If schools are significant places where the virus is passed on, then fewer Icelandic children (under 10) are exposed to that environment than UK children. If a child does not attend childcare the most likely place for them to pick up the virus would be from a parent - it thus makes sense that small children, at this early stage in the virus lifecycle, are less likely to have caught it than are the adults they live with.
Additionally, it is unlikely that a 9 year old's chance of catching CV is actually vastly different from an 11 year old - it is likely to be a sliding scale, with babies the least likely and 9 year olds almost as likely to catch it as 10/11 year olds. And it looks like 11 year olds have much the same chance as the general population. So the safest people are probably those who, certainly in Iceland, are too young to go to school.
Bascially, what this study tells me is that it is certainly not safe for those over the age of 10 to be attending school currently, from an infection point of view, and that whilst younger school aged children are safer, they are still at risk of infection and passing the infection on.