We built our volcano around a glass with baking soda in it. I also put in food dye; and yes, my children were wearing Sunday dresses (although not for long); and no, I wouldn't do the food dye again, but thanks for asking. Also it made the lava pink.
Then we piled up big rocks, little rocks, dirt, beans, more beans, split peas, more dirt. until we finally made a mound that reached the top of the glass. It was a small glass, and I was genuinely surprised how much stuff it took to build this little volcano. Keep in mind the dirt is about to be mud, so don't use any beans you hope to eat later.
Then the girls arranged their dinosaurs and played for awhile. This was funny, and I made a darling video of them: arranging the dinosaurs; explaining how the sharp teeth were coming into the valley; then Mommy made it rain in the volcano (vinegar), and it erupted! and the sharp teeth ran away from the great valley; hooray! Do it again!
Only the video didn't take. Boo. So you'll just have to imagine little girls delighted by pink lava, and sharp teeth running for their lives.
I put quite a lot of baking soda in our glass, so each time it fizzled out I would just throw in a bit more vinegar and it would go again. I think it erupted about 7 times. Afterward, the girls played in the muddy pink mess for a long time before moving to the bath. It was really sort of awesome and dazzling in a-hey, let's build a volcano-kind of way.