Raising a child helps you remember just how hard it was to learn certain things. Most kids pick up counting to ten without much problem (after they learn to talk). But grasping double-digit numbers (and beyond) is a greater conceptual challenge. Now you have to be able to count groups of ten (and then groups of a hundred, and so on) and represent them with digit placement. Later on, multiplication (and then exponents) build on a child’s earlier conceptual knowledge.
I’ve found that a pack of wooden cubes can help illustrate the relevant concepts nicely. When a child can see, for example, two sets of ten blocks, plus three extra blocks, the child can more-readily grasp the number 23. One issue I’ve seen is confusion about the number 23 versus the addition of 2 and 3; the difference is very easy to show with blocks. Of course the blocks are also really good for practicing simple addition and subtraction.
Although my five-year-old is not ready to tackle multiplication formally, we have started to work on it informally by setting up grids of blocks. He can see, visually, that two by two is four, three by three is nine, and so on.
The blocks are also excellent for illustrating exponents.
For this reason, I recommend getting a pack of at least a hundred blocks. They’re also fun just to build structures. I bought a pack of 500 (paid link) from Amazon that I’m happy with (the same link offers a selection of pack sizes). I consider these a key part of a math teacher’s toolbox.