The ATF disagrees with you:
http://www.atf.gov/firearms/faq/licensees-conduct-of-business.html#underage-employees
Notice how the ATF is interpreting the exceptions in 18 USC 922(x):
The <18 years of age employee may possess the firearm:
In the course of employment (working for the FFL which satisfies paragraph (i)) AND
Prior written consent of the parent or guardian (paragraph (ii)) AND
Who is not a prohibited person (another paragraph (ii) requirement) AND
The written consent is in the employees possession at all times (paragraph (iii)) AND
State law must not prohibit the juvenile from possessing the handgun or ammunition (paragraph iv).
If all 4 conditions did not have to be met, only "in the course of employment" AND "state law allows it" would be necessary.
Besides, if only (ii) or (iii) were required, than what would be the point of having paragraph (iii) at all?