Use a dedicated device
This tip relates to the previous advice about children and iPads.
Unless you only use your iPad POS system very occasionally, it makes sense to dedicate an iPad specifically to your point of sale needs.
In other words, you probably shouldn’t use the same iPad to play Candy Crush, take a bunch of photos and play with lots of apps. Get yourself another device to do these things. Not only does this mean you have one device dedicated to your point of sale, it can also mean you have a spare one to switch to if the main device is lost or stolen, or suffers a hardware failure.