What I don't understand is that people are "inconvenienced" by having to carry the letter, yet if it was their child that was abducted by a non-custodial parent or stranger, they may think differently. The officers are there to look out for these type of situations. The amount of questioning depends on each individual case/crossing.

Does it really matter if the letter is "notarized"? Probably not, but a quick call/contact by the BSO can be done to at least attempt to ensure if the proposed transit is genuine.