Firstly, introduce the idea that energy is a tool to think with…suitable for some tasks but not for others. There is a choice tree to navigate.
As a tool to think with, we have to exemplify the thinking to explain what the tool can and cannot do. So any attempt to teach 'the concept of energy', then to put the idea to work, results in understandable difficulties. (Always intimidate with Feynman: his introductory explanation is about a tool for thinking put to work, in a metaphorical story).
So, teach energy by choosing examples which illuminate by developing plausible, intelligible and fruitful insightful descriptions. Then children can learn to reason with the idea of energy in ways that do not distort its usage in the scientific community, by engaging with meaningful and purposeful descriptions. So what's a purposeful description? Back to the thermodynamicists for insight: energy calculations, first of all, tell us what can or cannot happen – which changes are or are not possible. (Later, you'll see how to reinterpret this core to develop other insights).
To create an energy description is to seek an insight – it's an inherently purposeful activity.