Number lines are a very early concept in mathematics. If it's taught properly it can actually be a very easy concept to understand and very interesting for kids too since its something graphical and kids love to learn things that they can see live.
So all you need to do is to draw a number line. On this number line, we have numbers from 0 to 10. Now, this number line can be used can be used to solve different problems. Addition can be made very easy with this number line. Its very simple.
For example, you have to find what adds to 7 to make 10. Now you simply need to do is to count starting from 7 and reaching 10 on the number line. The number of steps you take to reach 10 is actually the number that we need to add to 7 to make 10 which here is 3.
So, it’s just that simple. This was a brief lesson on solving the problem with the number line. As the kid practices more, he will learn more.