It is quite easy to count to 10, because we have ten fingers. It can be argued that counting to 20 is equally easy as we have ten toes as well, but in practice, getting them out every time to do math is just not as easy. For some kids, going above 10 in addition or any other tens (20, 30, 40 etc.) can be tricky in addition or subtraction. I will focus on going above 10 in this post.
We can of course use more hands. For some time now, I have counted with my now five year old up to 20 using her fingers and mine. This is a good tool and kids learning in a group can of course leverage this idea to count to quite high numbers. But at some point it is useful to be able to count not just with fingers. This tool is quite useful to help in this transition. Behold the wonderful measuring tape. I have a deluxe version where it can be collected in to a nice roll with just pressing a button. I paid perhaps 5 euros for it or something.
This is a step up from counting with fingers as we are using numbers. There is nothing concrete to count, but for a child familiar with addition and subtraction, this is quite doable. This also helps them to familiarise themselves with bigger numbers.
First just study the measuring tape together. Look at the numbers. See how high they go. Let them play with it however they see fit (that is safe for the house and everybody in it – kids come up with all kinds of suggestions).
When you have a mathematical problem that you want them to solve, or what would be even better, a problem they want to solve, introduce this tool. The process of counting with this goes as follows:
Lets say you want to add 6 and 3 together. Starting with something they can count with their fingers is a good idea, because then they can check with their fingers that they got it right. Find the number 6 together on the measuring tape (or you could use a ruler as well) and then take 3 steps forward toward the bigger numbers (one centimetre at a time, or inch if you are so inclined). You arrive at 9. Then ask the child to check with their fingers. If you need to subtract, then (6-3=3) take 3 steps backwards toward 0.
If your kid has a large amount of simple addition or subtraction, then they will learn this very quickly because they will use it a lot. I don’t have this yet, so we will use it intermittently and daughter will grasp it in her own time and by the time she will need it for school etc, she is quite prepared.