I think the problem is that "Krippe" means "manger", as in "baby Jesus in the manger". I don't have children, but I don't think the word "Krippe" is ever used to refer to the normal bed a child sleeps in. (Apart from that, "Krippe" also means crèche). From what I understand, crib can sometimes be used as an alternative for the biblical manger (?). That would explain why it's accepted but "cot" isn't.
See also: http://en.pons.eu/
Is this the link you mean? http://duolingo.com/#/comment/10694
To add to what I said above: it is not absolutely impossible that "Krippe" is used in the sense of "cot" (a bed a child sleeps in) in some regions. But this usage would be a regionalism and is not supported by any of the Standard German dictionaries I consulted.