I have no idea about when a trust is required to distribute its assets -- either ask a lawyer, or email Quicken.
I do know that while there are federal laws governing the minimum age at which an individual can
purchase a firearm
from an FFL, there are no
federal laws governing a minimum age at which to
possess a firearm.
So unless there are additional state laws where you live governing possession of firearms by minors, a six-year-old can legally inherit and become the registered owner of NFA items.
It then becomes a tricky issue in that the minor is the only one legally allowed to possess the item, but the minor's guardian has a responsibility to prevent use of said NFA item until the minor is old enough to use it responsibly. IMHO, this is another case where a bank safety-deposit box could be used effectively.
Just do what I did: Live long enough to see your kids grow up.