Whether it’s potential, heat and in this case, kinetic energy, mass is a factor of the energy.
For the same speed, the more mass, the more energy it has.
Same for gravitational potential energy. For the same height, the more mass, the more energy.
Same for heat energy. For the same temperature difference of the same material, the more mass the more heat energy transferred.
Even chemical energy like your lithium battery. the more mAH the battery has, the heavier it is.
As the kid gets into higher secondary level to study the formulas of all the different types of energy, there is always a “m” in the formula.
So I think the kid right now has to know is mass is one of the factor of energy. Which I think is the purpose of this question.