"When is their party?"
it's ridiculous to only accept 她们 and not 他们 ... they is not gender specific in English.
他们的派对是什么时候？ should be accepted. Unless "he/him/his" or "she/her/hers" is used in the English translation or a 女 or 男 is used to describe the person in an earlier phrase or sentence, 他 and 她 are interchangeable. I usually see "他们" as the default when there's no other details, but I'm all for gender equity and feminism and want to see "她们" used more frequently too. This is is still an error though, Duo. Reported 1/25/18
No, that sounds really weird to me. If possible the first position seems to be avoided for phrases containing question words. I think it has to do with the topic-comment structure of Chinese syntax (without going too much into it, new information is usually preferred to come late in sentences, and if you ask for something, that something inherently a new information).
Some of us do not to use the Word Bank but prefer to type our answers. Therefore, like others have mentioned, 他们 should be accepted