Originally Posted by EntropyControlSpecialist:
All of my preschoolers wear pants in the winter.
All of them get the wonderful, enriching experience of playing outside in the dirt, wood chips, etc. during recess.
All diapered children who sit down and play with woodchips have wood chips in their diapered area. I am unsure about the underwear wearing preschoolers as I do not need to change them, but they might as well.
I'm unsure what you're actual concern is. Do you believe the child care provider is opening the diaper and pouring woodchips in there? Do you believe they are not changing the child's diaper (is it soaking wet and or soiled every time you pick up?)?
How would I, personally, react? I'd be THRILLED that my child was involved in a program that actually took them outside and let them have wonderful experiences without worrying about keeping them in a "cleanliness bubble." Helicoptering is detrimental for children. Many research studies are available on that if you look them up. 
I agree. I also want to add that at 18 months, it's VERY possible that the child is actually putting mulch and sticks in their own diaper. I've found some crazy things in toddler diapers (and ears!)!