I'm building one. I technically started in October, but I never got too far. Since I'm on a break, I can continue the project.
It's on GitHub: https://github.com/ITVlab/Family-Calendar
Here you go: https://play.google.com/store/apps/details?id=news.androidtv.familycalendar
Very rough. Code's on GitHub too.
There isn't one that I've seen. However, while not ideal, you might be able to use this Chromecast app as a workaround: https://play.google.com/store/apps/details?id=com.rir.dashboardcast&hl=en
I haven't tried it on ATV, but I imagine it would work like any other cast app.