The defense explanation holds some water, but not much.
If a persons hands were clean is not necessarily relevant. Clean hands can leave very good prints on the substrate touched as the sweat is not blocked from the pores by dirt or grease etc. If the hands had been wiped with a handkerchief, rag or tissue paper etc, then there is minimal sweat on the surface of the skin to leave the required deposit on the paper.
Paper is an excellent substrate on which to develop latent prints as it is generally absorbent (thermal papers can be problematical). The sweat deposit from the donor is absorbed into the fibers of the paper and is generally protected. Whether the sweat deposit can be revealed by latent development techniques to disclose sufficient quantity and clarity of information to identify back to the donor will depend on a vast amount of variables. Such as, paucity of latent deposit, does the donor exude sufficient acids and oils that would react with the processing method employed, was the paper well handled by others before being handled by the donor you are trying to identify, was the paper collected and preserved using the correct methods for later laboratory examination, etc., etc.
From your posting you state that (during such a stressful time such as a kidnapping their hands would likely perspire somewhat and leave some mark). This would indicate that you may have a bias (I am not sure of course) and believe that the parents are in someway implicated in the crime. Of course it could also mean that the stress was caused by the loss of their child.
Whether they are guilty or not, some or all of the above mentioned factors may apply, as well as other explanations i.e., if the paper was previously well handled by other parties and subsequently handled by the donors (in this case the parents), their sweat deposit could be masked by the sweat deposits already present on the paper.
It should be no surprise for any fingerprint expert; that a persons prints may not developed sufficiently to individualize, after they had definitely touched the questioned item.
As you say, they may have left some mark on the paper, but if it was not sufficient to react with the processes employed and contained insufficient identifiable ridge characteristic detail to identify, then no firm conclusion can be made.
I was taught back in 1971, that when you examine any items for latent prints, you should always HOPE to find prints, never EXPECT to find them. Just because someone has touched an item, does not mean that you can prove it from a fingerprint perspective.
