What non-SMS non-Apple app can I use to "text" my younger kid on their iPad

Hi! My less-than-10-year-old has their own iPad (registered under my apple id) and wants to be able to “text” with the rest of the family. Most of us are not Apple people though: Android phones, using some combo of SMS, Discord, and Signal. The little one doesn’t have a phone, so I think that prevents us from signing them...