I don't see why it's difficult to train your child to understand if that number is bigger than that number then this number is one less... And if the number on the bottom is bigger just add the addend. And little more practice you can go from left to right instead of right to left. The idea is to make computation fast and easy.
It is amazing to me that we have fallen down this early on in the teaching of mathematics. This is just subtraction, it shouldn't be hard. If you want to make subtraction easy and fun for your kids just click the link. Supremely Simple Subtraction.