Would the parents need to sign a permit or something if their daughter is traveling to mexico with a relative?

My daughter is traveling with my mother to mexico for the summer, she's about to be yrs old, and I need to know what does she need besides the passport. Thanks!
My daughter is traveling with my mother to mexico for the summer, she's about to be 2 yrs old, and I need to know what does she need besides the passport. Thanks!
she's 2 yrs old

How old?

you just have to sign a consent form which most travel agencies have theres single parents or for both parents nothing else

Would have to be signed by both parents.

How old? If underage (and I assume she is) you should sign permission for her to travel with your mother. You might also want to add that your mother has permission to authorize emergency medical care if necessary, and have the document notarized (it proves you are the one writing the document and giving legal permission).

A signed, notarized letter saying that you grant permission for your daughter "Ashley", age 2, to travel with her grandmother "Mary Jones", between "June 1, 2007 and August 1, 2007" (add on a few extra days, just in case). Signed, your name
Signed, father's name
Notarized with seal

A signed consent for medical treatment suggested by another poster is a good idea, too.

You may also need a copy of the birth certificate to prove parentage. Photocopy the inside page of your daughter's passport with the numbers and the picture of your daughter. Give one copy to your mother to keep in a place separate from the original passport. You keep a copy of it at home. If the passport ever gets lost, then it will be much easier to get a replacement with that photocopy. Your mother should do the same for her passport also.
Also, I've never done it, but the American Embassy recommends notifying them of your location and itinerary when in Mexico. If there is ever an emergency while you are in Mexico, they can reach you. It might be a good idea, just in case.