This information comes from my favorite goto for reviews of any and all types of media, games or content that can impact children. It is called Common Sense Media and it has been around for a long time. Here are a few paragraphs to review if your child is spending a lot of time on this platform.
Is it safe?
"Partly due to the popularity of games such as Fortnite, Discord grew quickly (it now has over 100 million users -- a bit less than Snapchat but a lot less than Instagram's 400 million). It's also had safety problems, including being at the center of at least one serious crime against a teen and a human-trafficking ring. If they know where to look, teens can easily find mature content on its public channels. And its invite-only private servers also have become places where bad actors and even cybercriminals can come together to communicate. At the same time, teens can use the app without contacting anyone but their friends and avoid the unrestricted chat from other games and platforms for a more secure experience."
Who can join?
"Discord requires that users be at least 13 years old, although they do not verify users' age upon sign-up. Common Sense Media also recommends Discord users be at least 13 due to its open chat. Because it's all user-generated, there's plenty of inappropriate content, like swearing and graphic language and images (though it's entirely possible to belong to a group that forbids these). Discord also has "NSFW" channels and servers that require users to be 18 or older to join (but it's easy to click through the verification)."
Are there parental controls?
At this time, Discord doesn't offer parental controls, so there's no way for parents to restrict content or password-protect the privacy settings within the app.
The article also describes several ways to use Discord safely with certain privacy settings and parental monitoring. Type "bad experiences with discord" into Google and you will find many sources describing negative experiences, not just one or two.
How to control it?
Access to this app can be controlled through your Wi-Fi router and smart phone service providers Smart Family application. Communication is always the best first step.