Children can work with simple numbers worksheets from quite an early age and you will have greater success in getting them to work on the worksheets if you combine that learning work with something practical, or at least something they enjoy doing.
There are also worksheets that teach how to read. It includes the basic sounds each letter produce. Kids try to read the words displayed before them. In the First Alphabet worksheet, kids learn how to write the alphabet. And in the First Animals worksheet, kids try to recognize the animals in the picture and learn the names of these animals.