As part of the Digital Wellbeing project, sponsored by the GNOME Foundation, there is an initiative to redesign the Parental Controls to bring it on par with modern GNOME apps and implement new features such as Screen Time monitoring, Bedtime Schedule and Web Filtering.
Recently the ‘time is almost up’ notification inside child session has been implemented and merged in GNOME Shell, while preventing children from unlocking after their bedtime and allowing parents to extend their screen time are being polished up. You can track the progress at their merge requests at !3980 and 3999 respectively.