Any Canadian Citizen visit the US without passport or Visa (Till January 1, 2008 when Passports will be Required, unless Congress reverses this new law). The child is a Canadian Citizen, thus he can visit the US. The fact he or she is a child is NOT a factor on his or her right to visit the US. The only issue would be your right to have the child with you and verbal permission is all that is needed (Unless Canadian law requires something more if you have a child, not your own with you, something I doubt).
Now for practical concerns. If the child has a passport would be ideal, but not needed. I would also have a notarized statement from the parents saying they gave you permission to have their child with them. Such permission slip is NOT legally required, but it impresses bureaucrats (and the permission slip being notarized makes it look "legal"). Now the written permission slip is NOT needed, nor would I call it a "Legal Document", i.e. It will have NO legal validity. The note is just to show bureaucrats if any question comes up regarding your right to have the child. The note should say how long you are to have the child (i.e. for the next month etc).
Now, given the present hysteria about Security I would have passports for everyone in your group. If you do NOT have a passport (or if anyone in your group do NOT have passports) have birth Certificates for each member of the party (This is addition to any Driver's Licenses anyone in your group has). All of this is to show you are Canadian Citizens as opposed to citizen of some other country (If your children have School IDs I would take them along also.
For moreInformation (US State Department Site):
http://travelwithkids.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=travelwithkids&zu=http%3A%2F%2Ftravel.state.gov%2Ftravel%2Fwarnings_consular.html U.S. Department of State
Bureau of Consular Affairs
Washington, DC 20520
Consular Information Sheet
June 14, 2006
COUNTRY DESCRIPTION: Canada is a highly developed stable democracy. Tourist facilities are widely available except in northern and wilderness areas, where they are less developed and can be vast distances apart. Read the Department of State Background Notes on Canada for additional information.
ENTRY/EXIT REQUIREMENTS: All persons entering Canada may be required to present proof of citizenship and identity. U.S. citizens are encouraged to show a U.S. passport. If they do not have a passport, they should be prepared to provide a government-issued photo ID (e.g. Driver’s License) and proof of U.S. citizenship such as a U.S. birth certificate, naturalization certificate, or U.S. passport.
U.S. citizens entering Canada from a third country must have a valid U.S. passport. A visa is not required for U.S. citizens to visit Canada of up to 180 days. Anyone seeking to enter Canada for any purpose besides a visit, (e.g. to work, study or immigrate) should contact the Canadian Embassy or nearest consulate prior to travel. Anyone with a criminal record (including some misdemeanors such as Driving While Impaired (DWI) charges) may be excluded or removed from Canada, and should contact the Canadian Embassy or nearest Canadian consulate well in advance of any planned travel for further processing, which may take some time.
For further information on entry requirements, travelers may contact the Embassy of Canada at 501 Pennsylvania Avenue NW, Washington, D.C. 20001, tel. (202) 682-1740, or the Canadian consulates in Buffalo, Detroit, Los Angeles, New York or Seattle. The Canadian Embassy’s website is:
http://www.canadianembassy.org/.See Entry and Exit Requirements for more information pertaining to dual nationality and the prevention of international child abduction. Please refer to our Customs Information to learn more about customs regulations.
SAFETY AND SECURITY For the latest security information, Americans traveling abroad should regularly monitor the Department's Internet web site where the current Travel Warnings and Public Announcements, including the Worldwide Caution Public Announcement, can be found.
Up-to-date information on safety and security can also be obtained by calling 1-888-407-4747 toll free in the U.S., or for callers outside the U.S. and Canada, a regular toll-line at 1-202-501-4444. These numbers are available from 8:00 a.m. to 8:00 p.m. Eastern Time, Monday through Friday (except U.S. federal holidays).
The Department of State urges American citizens to take responsibility for their own personal security while traveling overseas. For general information about appropriate measures travelers can take to protect themselves in an overseas environment, see the Department of State’s pamphlet A Safe Trip Abroad.