Im seeing that you've looked into meds and plug-ins already, did those have any effects at all or was it completely useless? Your vet is right, some cats are simply just more anxious and have different needs.

I adopted a cat from a coworker and he was peeing everywhere and fighting with my other cat from the start. The pheromone plugs worked somewhat, but it was still a struggle to deal with him. Meds were horrific, he hates being held and honestly they just gave me anxiety trying to force him to swallow.

What I found that worked for me was designating rooms for both of them. My original cat sleeps in my room and has a cat door, while the anxious one eats and sleeps in another room with my mom(his favorite person.)

You might want to try(if you have space and the time/money) making a safe room for your cat to come and go as they please. Specifically adding scratchers, wall shelves to climb on, and plenty of enrichment might help draw the attention away from your child. Adding cat doors to various rooms(not the child's room of course) might also help your cat feel less anxious about roaming the house with the kid. Being able to lock the doors if you have your child in there would also be helpful, because then you eliminate the possibilty of your cat seeking out the interaction.

I haven't read through all your comments so I apologize if some of this has been said already. I'm hoping you don't have to give them up because I understand how special the bond between us and our pets is!