An issue with a lot of the reviews being left on here, as it seems, is parents complaining about different things that their child has been exposed to within the app itself. My first thing I'd like to address is that each user has a username and an ID number, making you hard to find unless you join a server/give out your name and ID, as both are needed to add people. Blocking a person and/or simply denying their friend requests are simple ways of handling this straight off the bat, and let's not get into the settings that you can put on your account to prevent people you don't have added, don't share a server with, etc, from messaging you.
Leading on from this, Discord does have a setting that will automatically block what you don't want to see- mostly NSFW things that I imagine many parents don't want their children being exposed to. However, as stated in Discords rules/terms of conditions, server owners are required to have a set channel for NSFW content that requires you to confirm your 18+ and safe to view. 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". Discord is a widely used platform, and it's quite frankly not possible to constantly go through hundreds of messages in several channels from one server at a time. If you'd like to sit there and attempt to do that, then be my guest.
Another time to point out would be that, upon being sent a Discord server invite link, it shows what it's called and how many people are on it. All you have to do to leave it is just right click it and press 'Leave Server'. If someone made that decision to do that, then that's their own fault. For your child to have joined a server based around sensitive topics you don't really want them to be around, they would have had to have looked one up and made that conscious decision to join it.
I know you may want to think that your child is sweet and innocent and weren't put up to it voluntarily, but that's quite frankly not true from someone whose been using the app for over a year now. I know it inside out unlike many of the reviews here, and I hope that this clears a few things up.