Curious to know what these are
These look like pieces of a disaggregated concretion from a sedimentary rock. A concretion is a chemical "growth" inside a rock. They can take different forms. Sometimes concretions crack due to dehydration or fluid infiltration, and minerals are deposited in the cracks. The yellowish coating may be such a mineral deposit; it looks like gypsum.
Another possibility for the first picture with the elongated feature, is petrified wood. Hard to tell, though, without seeing it in person.
There is a nice little fossil and mineral museum in Dickinson, it's located at 188 Museum Drive East just off I-94. The parents could bring their samples there and get an opinion.
If they are in Bismarck, they could inquire at the ND Geological Survey. Or, if they are in Minot, Grand Forks or Fargo, they could stop at the universities. We often look at samples that people bring in, so that would be fine.
Thank you so much for your time and information. Will pass along to my parents.