Some transit agencies do offer family passes. It’s done in different ways. Moose Jaw transit offers a discount if more than one pass is purchased per household (http://www.citymoosejaw.com/departments/transit/index.shtml)
Other cities offer family passes, but only for special events or evenings and weekends. See also http://www.catabus.com/ssfares.htm and Toronto transit.

Sadly, we don't seem to be a very family friendly country.