Use the concepts of kinetic energy and potential energy to describe the motion of a child on a swing. why does the child need a push from time to time?

The child needs a push from time to time because kinetic energy generates friction, witch takes kinetic energy and turns it into thermal energy, which floats away and can never be obtained again. So without the kinetic energy, and without someone pushing him/her, the child will only go slower and slower until he/she stops.