When my son was quite small, but old enough to eat "proper" food, I started making popcorn for him as a snack (unsalted, unsweetened, of course). Later, I taught him how to do it himself: put a couple of tablespoons of corn oil or sunflower oil in a saucepan with a tightly-fitting lid, and cover with popcorn kernels (readily available in your local supermarket). Turn up the heat and put the lid on. When the corn starts to pop, shake the pan (with lid intact) regularly so that all the kernels have a chance to pop.
Serve with seasalt. Or make a naughty toffee sauce!
Here's a nice recipe from the Waitrose website which offers a rather less naughty sweet dressing. And one from the BBC Good Food site....