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.
Kindergarten worksheets are often of much value in helping kindergarten children learn and re-inforce basic concepts in an interesting way. Small children usually like to do things rather than just read or listen. They also get bored rather easily. Therefore giving them well-designed, illustrated worksheets to do makes it easier and more fun for them to learn. Completing a worksheet also gives a child a great sense of fulfillment. In fact, not just for kindergarten, but even for older learners worksheets can form a valuable part of the learning process.
Quality vs Quantity,Some free worksheets are not good quality - the pictures are fuzzy, backgrounds print grey or speckled - and children tend to notice these things. If you are using the worksheets to educate your child, you may want to choose good quality worksheets that encourage your child to produce good quality work. After all, its a little difficult to ask your child to color within the lines and work neatly when the worksheet they are filling in hasnt done the same.