An update to this post
I've had my Matrix server running on my Cubieboard for the last three months. In that time, my kids have really taken to using the Riot app. The fact that both of my daughters use it is a real testament to the power of a good UX. I have it installed on my wife's phone as well, and while she doesn't actively chat, she does keep up with what we are doing in the Family room I have set up.
Currently working on getting them to use open social media as well (specifically Hubzilla ;) ). I am currently working building a channel to host the D&D game we are running. That should serve as a good intro to the system for them.