Specifically, the inability to install virtually any extensions onto Family Link-managed Chromebooks, something Google alleviated slightly this month with Chrome OS 80 allowing a handful of new approved extensions like Zoom and Hangouts, but a future update will open the parentally controlled floodgates and allow you to install any extension (via AndroidPolice).
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A child will be able to view the full Chrome Web Store and request the extensions that they would like to install, but under the current implementation, they need a parent to enter their password on the Chromebook locally in order to approve the request. That’s something that could certainly change by the time it makes it to the stable channel for the planned May 26th release of Chrome OS 83. Unfortunately, this is going to come with little time left in the school year for most students, but it’s an important update for home and school use that will allow parents to enable expanded usage for older children while still retaining Family Link controls.