Without an explicit contract I think it would be hard to argue they consented to commercial use of the photo, even if they accepted a payment at the time.
What else would they have thought they were getting paid for? A picture was taken, and they got paid; the only logical reason for them to be paid would be that there was some commercial value to the photo.
Does it really matter? The evidentiary standard is "preponderance of the evidence" so it's pretty much up to the defendant to prove that they had consent as long as the plaintiff can prove it was him on the cover. That's why some states even require the release to be in writing rather than a verbal agreement.
Well, I can almost guarantee that the first question that the defendant's counsel will ask in discovery is whether payment was accepted in connection with the photoshoot. The second question will be what the parent thought the payment was for.
A written agreement definitely helps to clarify the matter, but is neither necessary nor sufficient to guarantee a resolution.
