Messaging a public server (the server mods and admins might remove your child for their safety to prevent this from happening) can run the risk of someone messaging them privately and asking for personal information. A discussion around chat rooms is crucial, making sure they know not to join public servers.
Cyberbullying can occur on Discord. The app does encourage users to inform them of any abusive activity or behaviour by reporting and sending screenshots as evidence. Cyberbullies may then have their accounts banned.