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.
Before creating the worksheet for children, it is important to understand why the worksheet is being made. Is there a message to be conveyed? Can students record information that can be understood later? Is it being created to just teach a basic concept to little children? A well designed worksheet will make its objective clear. The different aspects that should influence the design of the worksheet are the age, ability and motivation of the students.