Hahaha, dude, they are way more contagious than lice. Washing your hands (which I do compulsively ever since working at a chain of porn stores as a young man) doesn't do a whole lot when you can get them by breathing, that's right they go airborne! How awesome. The eggs are so small, over 1,000 can fit on the head of a pin. So... kid running around your house? They are probably just flying all over the place.
It's terrible. You should see the fucking steam cleaning my house has gotten over the last 4 days.
One other stat - its estimated that at any given time, 40-50% of all children in the us are infected with pinworms, with most showing no symptoms and never even knowing. How is that possible? Well, by age 15 most adults have developed an immunity. So, chances are, you've had pinworms at some point in your life, possibly for years, and just never knew. It's gross as shit.