Gooey stuff is always pretty exciting for kids. This is called elephant toothpaste because, well, that's what it looks like. The experiment is fairly simple. The ingredients are:

• 125ml 6% Hydrogen Peroxide (ask at the chemist)
• 1 Sachet Dry Yeast (powder) + a few tablespoons of warm water
• Detergent
• Food colouring
• Empty bottle
• Funnel

You might want to wear gloves and goggles when handling the hydrogen peroxide. Add the hydrogen peroxide, a few drops of food colouring and a good squirt of detergent to the empty bottle, then swirl the mixture. Separately, combine the yeast with a few tablespoons of warm water and stir until mixed. Put the funnel in the top of the bottle, add the yeast mixture and watch the foam. The bottle will get a little hot (the reaction is exothermic). The reaction occurring is hydrogen peroxide decomposing into water and oxygen:2H2O2 → 2H2O + O2The bubbles are caused by the detergent capturing the oxygen. The foam is water and the colouring is made by the food colouring. The yeast is acting as a catalyst. The first few times I tried this, I was using yeast well past its used by date, and it didn't work very well, so make sure your yeast is relatively fresh. Yeast contains an enzyme called catalase. Catalase is found in nearly all organisms exposed to oxygen, and helps prevent cell damage caused by the by-products of oxygen metabolism. It actually catalyses this hydrogen peroxide reaction in living organisms.I did this outside on our grass, and as the by-product is water, it is safe to have on the lawn. If you are inside, make sure you have a tray to capture the toothpaste as it squeezes out. Wearing gloves minimises the chances of your skin coming in contact with hydrogen peroxide.  If you want to make the reaction more vigorous - in which case you have to wear proper safety gear - then you can use potassium iodide as the catalyst. But you might not have that lying around the house. There is some more info here.