This has been useful information, and between this and another post dealing with Minors in game teams, contracts and written agreements seem the safest way to start. To the point where I would ask why risk not having it.
I think the most likely issue would be publishing something on steam or Unity Store and making a few thousand bucks and having to provide a SSN number and bank account and receiving a 1099 from Unity or Valve.
Unlikely would be something epic like Facebook and the legal battles the creator had with his buddies at Harvard.
A simple plain English document explicitly spelling out what you all believe the arrangement to be (and then signed by everyone) is certainly far better than a verbal agreement or unsaid assumptions.
That sounds like a great suggestion. It could just say something like in the event the Project generates revenue...... the Parties will create additional terms conditions. (the last part isnt worded well but you get the idea)