Watch Together doesn’t depend on a Plex Pass membership. You might want to verify if your daughter is using a supported client app – IIRC, not all platforms support this feature (yet). Watching with managed users of your own Plex Home should be perfectly possible.
As far as I’m aware, the only area with a 1-minute limitation is the mobile clients. Those need to be unlocked – this can be done through a one-time activation… this activation is included if you have a Plex Pass membership. That being said… if she’s using your account and you got a Plex Pass, the mobile apps shouldn’t apply that 1-minute restriction.