TLDR: Google Family Link will not work, because it requires some very dangerous permissions, which are only granted when Google Play Services is a system app (not the case on Graphene).
Use something like TimeLimit.io (open-source, on F-Droid) to achieve similar control.
I don't think Google Family Link will work on Graphene, even after installing Sandboxed Google Play. That's because it messes with some system options, which no regular apps should ever have access to. Google Play Services being a system app (on stock Android) makes this work, because system apps have elevated permissions. This isn't the case on Graphene, because, as the name says, Sandboxed Google Play Services are installed as a user app, and don't have any more access than what you give them as a user app.