If you are going to spend time on science, though, please be wary of glib explanations that will confound a child's true understanding just a few years down the line. Language matters far more than facile explanations of the natural world. Unless you know what energy is, and I got to tell you that I do not, do not pretend a 7 year old can master this. Unless you can explain a concept accurately without using science jargon, do not pretend your lambs will get it. My students are amazed water comes out of flame, something easily demonstrated at any level of public education, yet accept that the Earth is round at face value, because you, the most powerful person in this child's life outside of family (and sadly occasionally including family), said so.