number one, give the delinquent cat a space that nobody can get to. not you, not your little one, and not the other cat. ideally this is when one cat is smaller than the other, simply make an opening where the larger cat can't enter. alternatively, if your cat is a percher, give it somewhere to perch. if they're a burrower, give them a new space to burrow. you can also do things like have the normal behaving cat wear a bell around their neck, and have your baby do the same. this way the cat will know the baby is coming and can leave the situation before getting mad.
number two, cats can get jealous. if you've had them since they were little, and now they're 10. they might think you're deliberately giving more attention to the new addition to the house. try to spend some time playing with them, go out of your way to feed them treats
number three, get some cat calming pheromone diffusers and put them throughout your house. they are fairly expensive, but they work. i have a rescue cat that was very, very aggressive towards my other cat. now she's still a jerk, but she's chill about it. like she will hiss and walk away rather than go on the offensive.
number four, if the cat is pooping outside of the box, they're trying to tell you something. could be sick, could be pissed, or most likely, could be extremely stressed out. have you taken them to the vet?
number five, Vetoquinol Zylkene. it's an over the counter mood stabilizer you can give your cat. if you take them to the vet and they determine that it's simply acts of aggression, they will most likely give them this to try. I would still consult with your vet prior to buying it on your own.
number six, exposure therapy. in tandem with some of the above, bring the cat into a room that it doesn't mind staying in and have it be a room with the pheromone diffusers. take some warm butter (NOT HOT) and dip your cats paws into it. the cat will start to be 100% focused on cleaning themselves, generally speaking they only clean when they are comfortable. as they are cleaning, sit in the room with your baby and don't acknowledge the cat. just show the cat they can be vulnerable in the presence of the baby. then work your way up to getting the cat okay with playing while the baby is nearby, etc. just show the cat the baby is not a threat or something that will impose on them.
number seven, do you have any close friends or family that the cat has gotten along with in the past? I would ask them to adopt the cat before taking it to a shelter. even a no-kill shelter, you have no idea their standards. you have no idea what kind of person will adopt your cat. I would give the cat to a shelter as a 100% last resort when you have tried everything.
Im not putting a bell on my baby.
Ive actually tried Feliway diffusers and cat attract powders for litter, but neither helped much.
I did bring the cat to the vet ~1.5 years ago when these things started and showed no signs of slowing down, and they didnt see anything physically wrong with the cat. I havent been to the vet for either of the cats since then though (have, instead, gone to Vetco for yearly vaccines).
Vetoquinol Zylkene: why not. I just bought it. Lets see if that helps.
As for having the baby with the cat- shes a toddler now. She absolutely will not sit still and let the cat get used to her lol. Shes been equally pissed off and angry at the cat; both of them do not like each other. The other cat and my child really likes each other though.