For younger children creating the sand volcano gives them opportunities for mark making in the sand getting ready to write, strengthening their muscles as they scoop, dig and shovel and learning patience as they create the perfect volcano shape.
As the children get older and more inquisitive you can start talking about chemical reactions with them. This may seem a complicated topic but they see reactions around them all day from baking cakes (an irreversible reaction) to freezing an ice lolly (a reversible reaction when it gets forgotten about)
Why not start by asking if they can now separate the vinegar from the bicarb and if not why not. You can then start asking them them if they can think of any similar reactions that can't be reversed.
We love volcanoes at Mini Professors. The children's reaction from the chemical reaction is always so wonderful to see and it is something that children never tire of.