Oukitel, by default, uses aggressive battery optimization which "kills" background processes, so we have to exempt Boomerang from this aggressive battery optimization. Thankfully, Oukitel does provide a method to have the permissions "stick."
From the child's device, go to Settings, Apps and Notifications, Battery Optimizer, there is a heading here that says not optimized with a down arrow. Click the down arrow to reveal the all apps option. Select that. Find Boomerang. Select don't optimize.