Install the tablets with the Google child account. During installation, remove all apps not needed to keep the tablet as clean as possible.
The end result is that the tablets are installed with some basic and required system apps. It will not be possible to install any other apps neither from Google Play or third parties/other sources without using the parent’s (admin’s) credentials.
It is possible to have multiple tablets on the same child account, so the same child account can be used for all the tablets.
Learn more in this Google article: Manage your child's Google Account with Family Link
Tip 1: Allow Google Drive to be installed. It is useful for sharing e.g desktop backgrounds/graphics that you want to “brand” your tablets with.
Tip 2: Install the Family link app on your admin device. From the app you can trace and monitor the tablets, you can “ring” them, and you can erase them.