This is from a huge and rigorous study in Korea, the kind of thing that can only be done when you have a massive, thorough, test-trace-isolate system and a huge, well-run public health service.
Through the epidemic, they kept track of every positive case, and traced down as many of the links as possible. With the way they got a handle on it, many pieces of that network are nearly complete. (The US doesn't have any of the pieces required to do something like this.) The result is that they can then look back to see who transmitted cases, where, and how.
The smaller, earlier studies that suggested children didn't infect adults much suffered from a lot of confounding variables - like kids being kept at home while adults went to work, shopping, etc. In those environments, it's not surprising that kids were rarely the index case in a household.
Actual research article here : https://wwwnc.cdc.gov/eid/article/26/10/20-1315_article
Meanwhile, the Missouri Gov wants your kids to bring COVID home from school : https://www.stltoday.com/lifestyles/health-med-fit/coronavirus/missouri-governor-s-comments-on-coronavirus-mccloskeys-raise-eyebrows/article_aeac1f42-9460-5605-9fe5-73ca3ca72b2f.html