"Passeport MTL" provides an contacts under the "Terms and Conditions" section of their website. The posting guidelines do not permit me to provide the information here.
Many destinations that are included mention "An admission," rather than multiple, however, there are some destinations that have room for more than one person such as a canoe, or quad bicycle.
You raise a good point for inquiring because the "MTL Passport" is likely based on an adult admissions and you might be able to save even more money by paying cash for the 'wee' little ones. For example, the Montreal Subway called "STM" reads:
"Up to five children 11 and under travel free when accompanied by an adult with a valid fare card."
Maybe call STM to verify that an "MTL Passport" qualifies. I submit that the "MTL Passport" is incredibly easy to 'tap' and use at the STM subway stations.
Usually under 3 years old is free admission. I’d look over the places you’d like to visit and ask first. If majority of them are free, I’d take the chance of paying out of pocket for one or two.
Most of the places on the pass probably have free admission for kids under a certain age, so check the age restrictions for each place on the pass you’re interested in going to. Depending on how many places offer free admission under 3 or 6, it’s more than likely cheaper to just pay out of pocket for them at any of the places that aren’t free for kids