You will need: Bicarbonate of soda, Vinegar, a spade.
Handy but not necessary; a pipette, 2 different colours of paint
Use your spade to make a little mud volcano shape in the garden. Make a little well in the top of the volcano and pour a little bicarb into the well.
If you have a pipette you might want to squirt some vinegar into a cup, so your child can use the pipette to suck up vinegar and squeeze it onto the bicarb. If you don't, your child will pour it straight onto the bicarb. Watch for the eruption.
Your child might want to repeat this a few times. If you mix the vinegar with paint first, you get a coloured volcano. You can use one colour and repeat again with a different colour to get stripes volcanos. This is also a good way to learn about colour mixing.
Note: If you do not have a garden you can do this experiment with a beaker, but you might want a tray underneath, as it can get a little messy!