It depends what you do with them. Often the most productive way of using a computer with children is for you to be with them, sharing something together, perhaps `helping' you with your work. There is a range of software packages targeted at young preschoolers. Look for ones that promote creative thinking skills and avoid ones that are often nothing more than glorified colouring in sheets that don't promote thinking skills and creativity. Playing with computers should not be an activity where children are alone at the computer for any lengthy periods of time. For preschool aged children, experimenting with a computer should mostly be about a social time with another adult.