Do I just put all, or itemize in the letter? Then what, have it notarized?
I would itemize, with one “bill of sale” or notice that the firearms had been given as a gift for each child. The point is to provide evidence created prior to any change in the law that legal ownership had already been transferred. Since the paper and writing will be old, that will be proof that your kids legally owned the guns before any new laws were passed if they ever need proof. Then when you transfer physical possession tell them to store the letters in their gun safes; no need to present the proof unless demanded.