I had messy in-laws too. Until my MIL died. She couldn't do much because she had a weak heart. The floors and surfaces were clean but the house was very dusty and food was just added onto of the deep freeze never rotated. When we had to help FIL move out there were about 15 chickens in the 2 deep freezes, one dating back to the mid 1980's

My advice is to grin and bear it. They might not be around for long and even if your DS touches the dirt then it will probably just boost his immune system anyhow. I do know how you feel though.