I have Gaxaly Note 10.1 with Android 5 and by default if I setup child account after device hard resetthere are no ability for enable adb and third party applications installation, just there are no this option in parents Family Link.

For enable it, need to setup device without google account, enable developer options, check, that adb working, and only after it setup google child account, in this case in parents Family Link there are options for enable and disable adb and third party applications installation.