As long as I can remember Dylan has loved numbers and at age three he could do simple adding and subtracting if we visualized it for him.  I would say to him if you and N were at the McDonald's play area and Sally, Sue and Steve showed up, how many kids would be at the play area and he would think about it and spit out the correct answer.  DH and I were so excited, we started expanding the visualizing mathematics as well as doing subtraction.  Our excitement, caused much excitement in Dylan in learning.

Now that he is four years old and knows how write his numbers, I gave him a workbook with some math problems in it such as 2 + 1 =   and he writes in the number.  We just started this last week and he loves it.  I stumbled upon a website called Lake Shore Learning.  In their resource section, there are flash cards that you can make up your own addition or subtraction, print them, cut them and then fold and tape/glue them to use them.  They make a fantastic learning tool and great to use when standing in line or waiting for your food to arrive at a restaurant.