OK - my credentials before I start (and to a certain extent, Vallhalla will be able to verify the credentials because she has worked with mutual people we know in rescue, who know me & my back ground very well) - I am a very, very experienced cat rescuer. I also have a LOT of domestic and wildcat (non domestic large cats, small cats & exotics, not just ferals) cat behaviour experience.
I'm not going to flame you OP, (although I am certainly not going to be gentle) because I think that is probably counter productive to the cats and it's THEIR lives I am worried about. To murder them because you are fed up is down-right unreasonable. That is what you are thinking of doing, BTW. Right, moving on...
The first thing that hits me, when you said that your cats are skittish is that they are stressed beyond belief, hence the urination problems. I am assuming, from what you say, that they have been checked medically by a vet and given the all clear? If that is the case...
Cats will pee around a territory to make themselves feel safe if they are stressed. Children being introduced - e.g. your DS, is a stress factor. If the cats don't get on, or they are being bullied/doing the bullying, that compounds the problem because they are trying to establish territory over the other cats. They do this with urine. Cats also have litter box avoidance issues due to being afraid of actually using the litter box - they may feel they don't want to use the same box as the bully cat, or they may be attacked whilst they are in the box. You get the idea.
As a long term solution, you need to get them on a Cats Protection or NO KILL rescue rehoming list, or rehomed in a more suitable home. Be HONEST about the peeing and spraying, the most likely thing is a stress issue relating to other cats and/or them being bullied by other cats.
I have taken on several "pissers" and because they are in a quiet, non-stressed environment, they have either stopped peeing inappropriately or my DH and I easily manage their issues because of what they do - e.g. one cat I have can't use a litter box, they frighten her because she has poor spacial awareness, so she poos in the middle of the bathroom floor. Solution to us, was to cover the floor with lino and just clean it up when it happens - I appreciate that others would not deal with that, but it's an e.g. It has also stopped her soiling elsewhere.
In the short term.
- Go to the vet and ask for some clomicalm or zylkene. It is an anti-anxietry drug which can help the cats. Clomicalm is only licensed for use in dogs, but my cat vet has used it very successfully and I have certainly had success with certain ones of my cats using it.
- Make sure you have AT LEAST 4 litter boxes around the house, if not more, in areas where they can use the boxes without being bullied. This means corners of the room, areas they soil regularly.
- Get a GOOD enzymic cleaner. I use "Urine-off" which is designed for cat pee. You can locate it on the web. It is essential that you clean up soiled spots properly, otherwise they will associate it as a latrine because using bleach does not clean it effectively.
- Feliway everywhere. use the plug ins in every room.
- Can you seperate the cats and give them their own territories and litter boxes? E.g. one in the living room, one in the bedroom etc?
They do not need to be PTS, they need help and that help may well be rehoming.