Make a mini sand volcano and bury a glass jar in the middle. Half fill with clear vinegar. Add several drops of food dye - or failing this try paint (which doesn't work as well), sprinkle in a little glitter (then hide the box from 4 yr olds other wise your house, car, chickens, etc will finish up all spangly). Add a good squirt of washing-up liquid and gently stir with a metal spoon. Add a heaped teaspoon of bicarbonate of soda and watch........
'The foam that pours out of the jar is formed by a simple chemical reaction. When the vinegar and bicarbonate of soda mix, they make lots of tiny bubbles of gas. The bubbles churn up the washing-up liquid, making lots of foam' (The Usborne Big Book of Science things to make and do)
This is also how we clean our loo. 1/4 cup vinegar, let sit for a few minutes, 1/4 cup bicarbonate of soda and a wee squirt of washing-up liquid (optional). The food color and glitter are also optional but if you want the kids to clean the toilet for you, ya gotta make it fun!
After all this heavy scientific learning, jump up onto the sandbox roof and pretend to shoot people!
I wish they wouldn't play with guns (It wasn't me that bought them) but even when they don't have any they use finger guns or find sticks in just the right shape! They will grow out of this, right?