I'm not going to give you a precise, general definition for energy. Instead we're just going to work with some examples of it, and of how you can put the concept to practical use. Once you've worked with it awhile, you can come up with your own definition, that makes sense to you. Hopefully, you'll also see how powerful and useful the concept of energy is. If the activity requires more energy than you have available, it won't happen.
Energy can exist in many different forms.
Show them how to calculate the amount of energy for a few cases, maybe kinetic and gravitational potential. Energy is conserved. But if you add everything up, making sure nothing slipped away unnoticed, the total amount of energy stays the same. However, some forms of energy are much more useful than others for the things you want to do. Gasoline stores energy in the form of chemical energy that can be burned in your car to provide the energy to make it move.