It does attack kids. Their symptoms aren’t as dangerous, though, and less often require hospitalization. Generally, their immune systems overwhelm the virus more quickly than the immune systems of adults.
Moreso, children rely more on their innate immune response as they haven’t encountered nearly as many patterns to build an effective adaptive immune response. Being a novel virus, only the innate immune response is effective. As we grow older, our adaptive immune response becomes more prevalent. Generally, this is better equipped to handle the bugs in our environment. However, with something completely foreign like this, it’s more of a hindrance. Those in their golden years gradually lose their innate response entirely, and thus the high mortality rate.