Word for the product of a challenge
Solution 1:
Well, if it were a contest, I would say "submission(s)" or "entry(-ies)" would be commonly used. I think this could work for a challenge as well. Also, one is said to answer a challenge, so perhaps "answer(s)" would meet your need.
Solution 2:
Just something that came to my mind, but I don't think they are anywhere near nicely fitting your case.
Work: an artistic creation, such as a painting, sculpture, or literary or musical composition; a work of art.
There seems no challenge explicitly emphasized in the definition of work.
Masterpiece: a creation that has been given much critical praise, especially one that is considered the greatest work of a person's career or to a work of outstanding creativity, skill or workmanship.
This word is apparently too much for describing your case, nonetheless it was the first word I thought of. By the way, the word "work" happens to be present in the definition of masterpiece above.
Solution 3:
In project management we'd call it a deliverable. However this doesn't sit comfortably with "challenge". I think something more ambitious like "achievement" maybe.
Solution 4:
Did you stop and think that your use of the word challenge is more or less synonymous with contest?
The only difference I can see is that contests have winners, runner-ups, and record the order in which the competitors finish, but challenges have those who pass or fail as opposed to a winner, runner-up, etc.
Merriam-Webster defines a challenge as "To confront or defy boldly." Synonymous with dare
Are you essentially daring these people to create something?
If so, then you can just call it whatever you want. Acts of creation. Poems. Arts. Submissions. Call it anything you want. But if it is a contest, you can then use the words with competition connotations with a little more tenacity.
Hope this helps, man.