It may be new but I feel that in the future it may very well replace programs like Skype. My daughter signed in to this "kid friendly" app while building on mine craft. So the only way that they are seeing this is by bad parenting or the child.
When our 19 year old came home from college, we checked the phone as we do with those that live in our house. we were horrified to see the different groups and the beyond mature content - language, graphics, etc.....
Also people can message each other and it doesn't pop on the screen like a text does so it is used to have "hidden" conversations.
Basically, anything goes--the most horrifying pornography & hate speech can be freely found & easily downloaded--as I found out with one of my sons.
If a 14 year old can find it, a company monitor could find it & delete it--since it is, after all, against their terms & conditions.
If your child has this for gaming they can be added to chats.
Be vigilant about checking this app if you allow it.
The thing that bothers me the most, is how parents think there is so much NSFW. Its going to be your child's or your fault for letting themselves/the parents get through to that group.
There is a setting for a NSFW channel if your in a public server, and if you wish to go to it, it will ask if your 18 or over, of course children can press yes and just go through it, but its the parents or the childs fault.
In that retrospect, your child made that decision to view that content, and is not at the fault of the server owner or Discord itself, because quoting Teamspeak- "This is Teamspeak, not Spyspeak".