When kids participate in making their meals, I find they are much more interested in eating them. This certainly applies to cooking, but it equally applies to growing vegetables and fruits - and making butter.
The whole idea of making butter came up when my four year old and I read Sarah Morton's Day, a great little book about the daily like of a Puritan girl living in Plymouth Plantation. One of the photographs in this book shows a woman churning butter. Consequently, my daughter and I had a conversation about how people used to make most of their own food, including their own butter.
Then I remembered you can make butter without a butter churn. I grabbed a canning jar from the cupboard (any jar with a screw on lid will work) and some heavy cream from the fridge. I let my daughter pour the cream into the jar and put a pinch of salt in with it. (The jar shouldn't be more than 3/4 full.) Then I secured the lid and told her to shake the jar until butter formed.
This project is great for when your kids have the wiggles, and if you have more than one child whose old enough to participate, all the better.
If you only want to make whipped cream, your child will need to shake just a few minutes. (And you'll want to add sugar, to taste.) If you shake longer, though, the cream separates and you can pour off the liquid (which is buttermilk) and cook with it or drink it. For every cup of cream you put in the jar, expect up to 1/2 cup of buttermilk.
If you want to explain the science, tell your child cream has globs of protein and fat. When he or she shakes the jar, the fat and protein begin sticking together. When they are completely stuck together, they are butter.