https://www.cnbc.com/2022/05/06/cdc-investigating-severe-hepatitis-in-children.html
It is up to 109 cases in the US across 24 states and Puerto Rico. 14% of cases needed liver transplants and 5 died.
More than half of the kids had confirmed adenovirus infections. Thus far the CDC has not be able to sequence a whole virus sample from any of the children, but many partial sequences were adenovirus type 41.
Still too early in the investigation to narrow the cause of the outbreak. The only thing that has been conclusively ruled out is that it isn't caused by the coronavirus vaccine as nearly all of the cases have been in unvaccinated children.
The CDC stressed as well this is still extremely rare. They aren't seeing a rise in hepatitis cases among children in their syndromic surveillance.
So what are the possible causes?
Viruses, previous viruses causing a weird immune response to a viral infection, viral coinfections, environmental or toxin exposure, etc. Everything is potentially possible at this moment, save for the coronavirus vaccine. But environmental/toxin exposure seem less likely than viral at this point given such wide geographic distribution