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.
A young child may not be able to write or read more than a few words. Worksheets should be created keeping these factors in mind. When you buy worksheets for your children, look for how the concept is explained. Is it pictorial or is it just a collection of words? A pictorial worksheet will hold the attention of a child more than just a combination of words.