As the girl at the highest point of the swing begins to descend, her potential energy is at its maximum due to her elevated position. As she swings downwards, this potential energy is converted into kinetic energy, which is the energy of motion.
Therefore, the best way to describe the changes in energy is:
"As the girl descends, her potential energy decreases while her kinetic energy increases, keeping the total mechanical energy (ignoring air resistance and friction) relatively constant."
This reflects the principle of conservation of energy where energy is transformed from one form to another.