Honestly, any secure messaging app would suffice! Maybe minus Signal since that needs a phone number currently.
I use an XMPP server to do messaging for my kid without a “real phone”.
Honestly, with lineage it’d be pretty locked down as they couldn’t do Google play apps for major social media, but I guess it depends on how old and knowledgeable they’d be about loading micro G for that stuff.
Also, iirc, you can use an MDM with Lineage OS bc it’s still Android at its core, which could run Android enterprise, and Manage Engine does full MDM for free under 25 licenses/devices. Might be worth a test to deploy apps that way. I’ve done that for regular Android devices when my oldest wanted more freedom without me having to utilize Google all the time.
