The battery life of a tablet can determine just how long it will be usable away from a source of power. This life does vary a great deal depending on what use the tablet is put to.
The Fire 7 Kids Edition Tablet has a listed battery life of up to 8 hours for mixed use and the Fire HD 8 Kids Edition has a listed battery life of up to 10 hours.
Certain things will use up the battery faster. Watching videos, for example, tends to be a greater strain on the battery than reading a book.
There are methods to help reduce the amount of power used. WiFi and Bluetooth both use up a fair bit of power and if there is no WiFi available and Bluetooth isn’t going to be used it makes sense to turn them off. Closing apps that aren’t being used is another method.
If the Fire tablet is going to be used in a car at any point, it can also make sense to purchase an in-car charger.