I just spoke with the Department of State yesterday over this very issue. I'm divorced, and my ex and I live too far apart for a notarized letter to be really do-able, and more inconvenience than I'm willing to ask him for. I don't have a copy of my divorce decree showing custody, because they don't give a copy to you. (Not for free, anyway -- it's a dollar a page, and 50+ pages.)
The gentleman with the Dept. of State was very nice and said that just a plain old letter, in absence of the decree itself, would be just fine. I had also called about the birth certificate issue, since my son's is an official certified copy but has no raised seal (just an engraved border.) He said that was fine too, that there are states where the birth certficates don't have raised seals.
Anyways, if your daughter has a copy of her divorce decree showing she has custody, then that is officially all she needs. If she's like me and doesn't, then you might want to just try to get a note from the dad just to be sure.
The US/Canada border is *way* stricter on that kind of stuff than any of the many other borders I've crossed. I guess that Canada is where most non-custodial parents tend to bolt to if they're trying to escape with their kids, or something. I was almost denied entry several years ago because I didn't have a note from my ex. Luckily, they were able to reach him by phone to verify that he knew where we were. Otherwise, I'd have been S.O.L.!
-gina-