A child is sliding down the slide at the playground is the mechanical energy conserved why or why not

No. This is due to friction. The potential energy of the kid at the top of the slide is E=mgh. But at the bottom, while the potential energy has been converted to mostly kinetic energy, some has been lost to friction. This results in heating up the slide as well as the kid. Ultimately, energy is conserved but potential energy is converted to kinetic energy and heat energy. Only the kinetic part is called mechanical energy. The heat part is called thermal energy.