How to unlock non owner account after reboot?
It seems like by definition the primary user (here called "user 0") has to log in by definition as the first user. The documentation on the File Based Encryption (FBE) does not give many details on the question why:


  Each user in a multi-user environment gets a separate encryption key. Every user gets two keys: a DE and a CE key. User 0 must log into the device first as it is a special user. This is pertinent for Device Administration uses.


Source: https://source.android.com/security/encryption/file-based#supporting-multiple-users

Hence I don't see a chance to change this behavior, you can only teach your children not to reboot the tablet and not to switch it off. In air plane mode with really disabled Wifi the battery loss should be minimal so that the tablet can stay on even when it is not used for some days.