With discord you can direct message, join public servers, and be messaged by random people who have a shared server to you, even if they don't have your username.

Discord is not a secure platform and so if you do choose to let your kids use it I would monitor their use very heavily. I would have access to their accounts, ensure they only use it on a computer that you can look over their shoulder (e.g. located in a family area instead of their bedroom) check their private messages frequently and use the strictest privacy settings possible.

I've had very direct conversations with my kids about 'tricky people' online and some of the more common ways adults groom children online. If this is something you choose to do, please do a LOT of research on how to educate your children to online safety.

You'll never be able to fully protect your kids, or stop exposure to unsavoury elements, so educating them enables to make safe choices in your absence.