IS there a way to create a relationship using a list user field as the reference field in a child table?

Short Answer: Not really

Medium Answer:  You could have those child records duplicate / copied and then just change the Crew Member.

Medium Answer 2: YOu could make the child table for work entries 'editable' on the form, and have it only have the dropdown, and the rest of the values are lookups from the project or the crew member.  i.e. The work details would actually be up a level, and just sent down for reporting purposes.

Long Solution / Complicated:
Use the Multi-select field on the parent level, and then run a script that creates child records for each of the users, and relates them appropriately.