Here is my solution:

Provide easy to use on-device content filtering tools so parents can easily control what their children can access (there are a few ways to do this through law, like requiring it from OS providers or ISPs or just writing these tools directly).

To make it easy, Discord can provide their services under both adults.discord.com and minors.discord.com so parents can more easily block only the 18+ version of Discord.

Require personal responsibility from parents to decide what is appropriate for their child.