I would call a 'crib' a 'cot'. I feel like crib may be an americanism? But I think 'cot' should be accepted anyway?
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/
I totally agree with Katherle. We had an extensive in detail discussion of the topic some months ago here in the forums. But I can't find it anymore. duolingo's search function isn't working very well. If someone has the link, it would be nice to post it here.
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.
Strange, when I did the same it didn't show me any hits. Maybe I did something wrong ;-)