When the child's phone is at 20%, 10% and 5% and not charging, you get a notification. If the child is away from the “Home” place and the battery looks unlikely to last the trip, you get an urgent “… phone may run out on the way. Call now while it is online”. It is an estimate, not a promise.
The 20, 10 and 5% warnings are free. “May run out on the way” needs the subscription.