The best way to get kids attention is to start with something that defines intuition, and really focus the discussion on that to begin. Example: we all know that when you cool a substance, it goes from gas to liquid to solid. When you heat it up, it goes from solid to liquid to gas. Look at the noble egg—goes in the pan as a liquid, and as it heats.........wait, ok, well that's a bad example. We all know that when you something turns into a solid, it gets denser, and we know that dense things sink in less dense things, just like ice....wait, another bad example, darnit. On the ice one, some kids will get hooked just because it behaves opposite, other kids will find the consequences of this fact more interesting (that life could not have formed if lakes froze bottom-up, etc).
Each time you want to explain some new principle, the way to set it up is when this new principle is juxtaposed against some other principle the kids already know, but wins out because it's more significant. This is how many discoveries are made in science and why scientists consider the problems intriguing, why should kids be any different? Trying to explain these anomalies is where the aether theory and relativity came from, the photoelectric effect, discovery of the 4 forces (gravity pulls a feather and a hammer the same, how electricty & magnetism are manifestations of the same force, discovery of the weak and strong forces too), superfluidity, the transistor, etc. I believe it's also a good idea to introduce kids to these advanced topics early on, without delving too much at least explain the motivation behind them by telling them the problems they can answer.