A highly recommended mine that you can explore on your own. It's only 3°C in the mine, so you should dress warmly even in midsummer... or change accordingly on site. It's already cold from the entrance.
You can get a bit dirty :-) Rubber boots (highly recommended), helmets, flashlights and jackets are available on site (included in the price)
The facility is very beautiful and in great condition. The treasure maps are waiting for the children, which gave them a little motivation boost!
Overall, it's not cheap but it's worth the money for this trip! It's best to get the tickets in advance because card payments aren't always possible on site: weak internet reception...