My wife and I are planning a trip at the end of the month from the US
to Canada. My grand daughter, who is 3, will be traveling with us. Her
mother is writing a note giving us permission to take her. Will this
be enough, or will we need something else?

Hi Summer95, 

The letter from your daughter should be enough, at least according to
the Canadian Border Service.  But, just to complicate things a bit,
the US Department of State advises that your permission letter should
be notarized (direct quote from the document linked below):

Due to international concern over child abduction, single parents,
grandparents, or guardians traveling with children often need proof of
custody or notarized letters from the other parent authorizing travel.
(This is in addition to proof of citizenship as explained above.) Any
person under the age of 18 and traveling alone should carry a letter
from his/her parent or guardian authorizing the trip. Travelers
without such documentation may experience delays at the port of entry.

This comes from this page (which is cached by Google as the page
doesn't seem to load from the main server at the time of this
writing): http://64.233.161.104/search?q=cache:5tS_N2fKAG4J:travel.state.gov/tips_canada.html+canadian+border+relatives+taking+children&hl=en

However, this advisory is for travel to Canada AND Mexico.  Mexico
apparently requires notarized documents, while Canada does not. The
Canadian Border Services Agency states that (again, quoting directly):

Adults who are not parents or guardians should have the children's
identification as well as written permission from the parents or
guardians to supervise the children. The permission letter should
include addresses and telephone numbers where the parents or guardians
can be reached.

That paragraph comes from this page:
http://www.cbsa-asfc.gc.ca/E/pub/cp/rc4044/rc4044-e.html#P144_3730

You may want to contact Canadian Border Services to double check
before you go. Their general information line can be reached at (204)
983-3500 or (506) 636-5064. Unfortunately toll-free access is
available only from within Canada.

I hope this provides sufficient information.  Enjoy your vacation!

Hibiscus

Very helpful. Thanks.