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 ABC worksheets should have different activities to help children identify the various letters of the alphabet. The activities may involve very simple things like colouring, ticking, drawing a line to match items etc. Using attractive illustrations and cartoon characters would make it more fun for children. The activities should be graded, i.e initial activities should be very simple and easy (but should be fun with good pictures etc, so as to interest the child); later worksheets may involve a little bit more work.