I found this on YouTube and it looks super cool for you and your kids to do.
Here's the how-to from the video:
"This is easy, home made and readily available in your own kitchen. My daughter Haylee is pouring 1/2 cup of vegetable oil with 2 tablespoons water 8/10 drops of food coloring and 1 tablet of alka seltzer. We did this at the dinner table right before lunch, and it takes about 8 minutes from start to finish. It is super easy to do and clean up. It doesn't make a mess. We suggest using a different container to empty out the cup of oil when cleaning up and rinsing with soap and hot water."