Imagine that your local local movie theatre showed everything from the latest Pixar hit to film pornography. A single ticket allows anyone--even children---to enter the front doors and then choose a theater room. Which one shows porn? That's not clear until the movie starts, but somebody is handing out candy for entering theater 9.
Discord is like that. Some servers are well-supervised and kid-friendly, and kids can even join private servers with only their friends. Some are full of the darkest extremes of humanity, and they all look alike from the outside. Many kids join Discord to talk to an approved group, which is no more risky than any other group chat.