Skans is rightly concerned about the unborn successor trustee issue, but your attorney should be easily able to sort that out with you, as it is based on your state's trust laws.
As for the authorized users, at the recent National Firearms Law Seminar held at the St. Louis NRA Convention, William J. Ryan, Esq. from the BATFE Office of the Chief Counsel gave the lecture on NFA Trusts. He clarified the BATFE's position on possession of NFA items by saying that the Trustees are the ones allowed to be in possession. He said:
"The person named as the trustee would be the lawful possessor of the NFA firearms. Clearly the piece of paper itself can't possess it, someone needs to possess it, and that would be the trustee."
He went on to say that the beneficiaries have a future interest, and do not have present possessory rights unless your state law dictates otherwise. State law controls in these matters.
Ask your attorney about any problems in the area of constructive possession when people not named as trustees have access to the safe where the items are stored. When the future kids become old enough to have the combination, this may need to be addressed.