We have found that Google's Family Link applies a block to changing the preference of the Install unknown apps setting. We require this setting to be changed temporarily when installing our version from our website in order to keep our call and text monitoring features working. Here's how to disable this block from your parent device's Family Link app.
On your parent phone, open the Family Link app, select your child, go the Phone > Device pane > SETTINGS > Apps from unknown sources.
This will then allow the installation of Boomerang's version from our website.