I want to remind you that your lease states “The Property shall be used by Tenant only as a private residence and for occupancy of no more than the 1 adult identified on this lease and 3 children.

What do I charge them in the event the extra three people (2 adults, 1 baby) stay for longer than 5 days? 

adults=2&children=...

Hi there and welcome to BP,You will have to check to check the local codes first- I invest in a college town and they do not allow more than 3 unrelated adults to reside together unless you have different zoning.  

They are adults. 

We've never thought we needed to set a cap on occupancy but two adults, two near adults and a grade schooler seems very tight and we're not really ready to have huge water/sewer bills associated with what would be a lot of people in this property. 

Other times psychopaths get in as boyfriends or adult kids coming back home, etc.

The group @Percy N. mentioned is DIG http://digonline.org/The horror stories of others are the sort of things you want to pick up vicariously - and they were entertaining to a certain extent.

This thread has been super valuable... and a bit entertaining!

Conveniently located close to schools, shopping, dining, entertainment, and all the amenities of Mount Vernon and Skagit Valley.