It is easy to see how this add value to your kids life, since most kids love a little treasure hunt. :)

and in time it will also add knowledge.
For example as you say, you live near a volcano, and then it usually is pretty easy to find volcanic glass / Obsidian.
Obsidian might look kind of boring just to look at, but if one add a history to it and how much it has meant for people in the past, because of all the different things it was used for, it also get a historical value. :)

Epidote is also another mineral that often is common to find in volcanic stone.
And some times you find perfect and clear crystals of it, and those are very beautiful to, and many collectors love them because of their beauty. :)

I hope your kids will find many crystals and that this will make their interest in this grow. :)