Do choose worksheets that are appropriate for the childs level, since children can get discouraged easily, if they are unable to perform the activity. Well illustrated worksheets with cartoons are also more likely to appeal to children. Worksheets that use common situations children come across at home, school, in the market place etc and that use common objects known to children would be more relevant.
Another thing to look out for is what the pupil will need to solve the worksheets? Does the worksheet require the use of crayons? Does it require other things like a pair of scissors, glue and so on? Before you buy worksheets, make sure to check if they have been created to suit the geographical location that you reside in. The language and usage of words differs from country to country. It is no point buying a worksheet which is designed for children in the US for children residing in India. Also see if the worksheets involve just one way of teaching or multiple ways. Do the worksheets involve short assessments? Does it have some activity built in; does it involve elements from the childs surroundings?