Care should be taken to give children worksheets that they are capable of doing. This involves understanding and monitoring the child continually, since the level of attainment of different children would often be quite different. The worksheet should challenge the child but not overwhelm her. If the worksheet is too easy or too repetitive, it may bore the child and she would not be happy. If the activity is too difficult it would frustrate her and she would not like to take up more sheets. The children should love to do the worksheets; they should not be thrust upon them. Also doing only worksheets alone repeatedly would not be very productive. You should have a range of physical games and activities as well that would reinforce the concepts learnt.
From a teachers perspective our competition is tough. Passing out a handout of 30 problems that are all in a format of 534x25= is not as stimulating in the students eyes as playing games such as Grand Theft Auto and Resident Evil. Granted, that will always be a tough uphill battle for math to win out over most video games, but the point is, students today are much more immersed in technology than ever before. So even if you need to pass out a math worksheet to review concepts and formulas, it will greatly benefit your cause if you design the worksheet to be as stimulating as possible.