Standard equipment list given to parents. No mention of weight limits.
http://www.scouts.ca/media/documents/Equipmentlist.doc

Duty of Care...
http://www.scouts.ca/dnn/ForParents/DutyofCare/tabid/149/Default.aspx

Duty of Care

The mission of Scouting is to contribute to the education of young people, through a value system based on the Scout Promise and Law, to help build a better world where people are self-fulfilled as individuals and play a constructive role in society.

The Code of Conduct is expected of all adults who volunteer within Scouts Canada, recognizing that at all times they are expected to act responsibly and exercise a "Duty of Care" to the youth members.


These are the safe limits according to the medical profession:
http://z.about.com/d/ergonomics/1/0/w/-/-/-/bpweightchart.jpg

According to that chart, my 9 year old daughter should carry no more than 5 pounds in a backpack.