Translation:When is their party?
I think that's fine too. Technically "what time" is a bit more specific than "when" so I think yours is alittle more like "... 派对几点 ... " But yeah, pretty much the same.
For what it's worth, 派对 is a transliteration of the English word "party" so it's kinda easy to remember.
"When will their party be" should be accepted. "When is their party" is semantically in the future, just like "Tomorrow is Monday". Reported it.
But that's not really what the question is in Chinese. The literal translation is, "Their party is what time?" The point of these exercises is to help users to learn vocabulary, not to parse semantics.