First and foremost, I start with the assumption in line with Catholic Doctrine that angels and demons do exist and can have influence upon our material world, if you disagree you are out of line with the Church and will find no value in my arguments.
From my understanding, the issue can be more complicated than we like to think. Please read this to the end to get my point, don’t jump to conclusions.
As shown by the few that simply laughed at your inquiry, we don’t want to believe that anything fictional can possibly have negative repercussions. As shown by your sources, some of us like to find enemies in everything we come across, which may not actually be the worst position to take. (better to be over-prepared for an enemy than unaware of him)
Based on a few pieces of information which I have come across (sources I can’t remember so feel free to take it with a grain of salt) the snake in the garden comes from a Hebrew word that can also be translated “dragon.” For quite a long time in western mythology, dragons were an evil and demonic representation.
Believe it or not, even playing with fictional representations of demons can open you up to some less than positive influences. It is a good idea not to pretend to be a demon or to pretend to work with a demon because you must be careful of just how much you “will” to involve yourself with them. Demons look for even the slightest form of consent to take advantage of some weakness of ours.
Given that, it is possible to understand, even with disagreement, why some people, particularly staunch traditionalists, might have a problem with good dragons. Now, this all has to be taken into consideration as well as the fact that mythological representation has shifted. As well shown in ancient eastern cultures (who have long had “good” dragons in their fiction), it is quite possible to have a similar symbol mean exactly the opposite. Is it possible for a traditionally demonic symbol to be changed? I think so. So long as the context and intent are clearly not demonic, there is no reason to fear demonic involvement simply because its about dragons.
All that being said, I caution anyone with children to stress the difference between fiction and reality. Make sure your kids do not search for dragons because there may be those out there who will use such curiosity to lead children into interacting with dragons meant to be taken as demons.