How do I teach my children good manners and character when interacting online?

Teach your children that Islamic character does not come off when they are behind a screen; the person at the other end is still a human being with feelings. The Prophet ﷺ described a Muslim as “the one from whose tongue and hand the other Muslims are safe” (Sahih al-Bukhari 10), and the fingers that type are part of that hand. Teach them not to write what they would not dare say to someone's face, not to spread other people's secrets or photos, and not to join in group mockery. Encourage them to spread good and to help, not to seek attention. Set an example through the way you yourself interact online. Check in now and then, not to spy, but to guide. See educating the family.