M.L.
Here are some tips, for what they're worth:
Dogs have family. Cats have staff. You need to know that.
That said, cats can be affectionate. They all have different personalities, but generally a cat will be affectionate with you on its own terms. That's one way in which they are different from dogs. We have had very cuddly ones. We've had others who absolutely hate being picked up, but will jump on a handy lap if it's their idea. Since we've never had to *choose* a cat - God sends them to our front door! - we've had all sorts of personalities and have come to like them all.
It's natural for a cat to scratch on something. I've found that having a cat perch or two works well. They're not cheap, but they last for years. They're those tall climbing things covered with carpeting. Kitty can scratch on them, jump up and down them, and sit on the top perch to survey the neighborhood. All those - scratching, jumping, and surveying - are things all cats love to do. I also have scratching boxes with corrugation board inside them. I add a little catnip to those boxes, and the cats are happy to stay away from the arms of the sofa. Scratchers have to be sturdily made; those two-foot upright scratching posts you see in stores aren't worth the money, because any cat bigger than a kitten will knock them over and then they aren't much good.
You toilet train a kitty by giving it a litter box. You do have to keep it clean, but that's not hard.
I'm assuming this kitty will be an indoor cat. I would keep a kitty off the balcony, period. I'm sure you could add to the balcony fencing, but it might have to be right up to the top. Some cats can jump up to five times their own height (or something like that). I might certainly put a cover on the fish tank. Our first cat, McGee, shared our home with us, a small dog, and a couple of gerbils who lived in an old fish tank. I covered the tank with a sturdy piece of screening, and his favorite place to sit was on that screening - not to get the gerbils but just to watch them. But he was not an aggressive kitty.
Thank about teaching your cat to be comfortable in a transport crate. You'll need to have one to take it to the vet or anywhere else. A cat loose in a car can be a major distraction.
You might want to talk to your parents' dog's vet about what else you need to know before you get a cat - especially in the health area.
There are plenty of cats *desperate* for homes. Consider getting an older cat (say, a year or older), since your DS is pretty young. Our humane society won't place baby kittens or puppies in families with young children.