For the purposes of entering Canada, the duration of the trip does not make a significant difference for the entry requirements. Regardless of the length of your trip, everyone must present a negative COVID test (taken within the prior 72 hours) upon entry; and all members of your family age 12 or older must present proof of vaccination (assuming you are not Canadian citizens.)
You are also required to have a "quarantine plan", which details what you will do if your vaccination details are deemed not to be in order by the border agent. If this occurs, you can simply turn around and go home; see this answer for how to indicate this in the ArriveCAN app. Alternately, you could continue in to Canada; but your family would then have to isolate for 14 days, with no visits from guests (which is why this is not a great option if you're planning on a trip shorter than 14 days.)
Note that your unvaccinated children will be required to test for COVID before arrival and upon arrival (if selected), just like the rest of the family. They will also be subject to a list of restrictions on their activity while in Canada:
For the next 14 days, the children must also: ...
- not attend large or crowded settings, indoors or outdoors, such as an amusement park or sporting event
- not take buses, subways, trains or other crowded transportation
...
- stay in a place that allows the child to avoid all contact with any person that:
- has an underlying medical condition that makes the person susceptible to complications related to COVID-19
- has a compromised immune system from a medical condition or treatment; or
- is 65 years of age or older
- limit contact with others:
- remain with their fully vaccinated parent or guardian, as much as possible
- wear a mask and physically distance when in contact with non-household members
In particular, your 6- and 7-year-olds would have to wear masks and physically distance around your sister-in-law (unless she is older or has medical conditions which make her susceptible to COVID-19, in which case they would have to avoid all contact with her), and they would not be allowed to be anywhere there are crowds present.
Finally, note that travel requirements are subject to sudden changes; either Canada or the United States could change their requirements tomorrow, particularly with the uncertainty stemming from the emergence of the omicron variant. You should watch both countries' requirements carefully, and be ready to modify your plans on short notice. And please do seriously consider giving your younger children the protection that the vaccine provides.
