There is some evidence schools, and young children, are not spreading the disease. In SG, which did extensive contact tracing early on, there were no clusters associated with schools (apart from one where the cluster was in staff who had minimal contact with students). The early super spreader event from an SG conference resulted in a Covid-19 positive child, and not one case was attributed to this person. Anecdotal, not rigorous, but still evidence.
There's emerging data in the UK of a syndrome that's connected to covid-19 in children (but it may not be covid). Whatever it is it's not common, but a small percentage across a large population is a large number.
https://www.theguardian.com/world/2020/apr/21/boy-with-covid...