Making homemade bath bombs is a great project to involve the kids, especially if you are stuck inside and are out of ideas to keep them active. These bath bombs make an amazing project to do inside. Most of the ingredients can be found in your kitchen and bathroom cupboard and are completely versatile. You have to purchase some essential oils. There is a very wide variety of scents from which to choose. The kids will come up with amazing ways to combine scents and colors.
Here are ingredients you will need:
1 cup baking soda 
1/2 to 3/4 cup sea salt or Epsom salt
3/4 cup cornstarch
1/2 cup citric acid
2 tbsp of your favorite oil (almond oil) e.g.
2 tsp witch hazel (or water, keeping in mind that witch hazel is superior)
1 tsp vanilla extract (or water)
30-35 drops of essential oils 
Food coloring
Combine oil of choice and essential (fragrance) oil and food coloring into baking soda and stir well.
Add dry salt, citric acid, and cornstarch. Stir well. Dried herbs optional.
Spritz witch hazel slowly on mixture and mix well. Do this until mixture will hold together when squeezed and not crumble. Add more witch hazel if too dry.
Spoon mixture into molds or your choice of containers. Press firmly and wait a couple days. Mixture will hardened and expand. Occasionally press down mixture firmly into containers. Wait a couple of days for bombs to dry. Remove and store in covered container.
There are many variations online so take a few minutes to check out the many ideas available and of course, use your own imagination.