Just adding my experiences to the thread.
I have two indoor-only cats. When we just had one, she never had any interest in going outside, since I never let her out even once. She was bought at a pet store (she was a “gift” from a friend… Sigh that’s another issue altogether, but I took her in). This same friend kept trying to “let” her go out, since she thought having cats indoors waas just “so cruel”. Ah. Yeah. Okay. The cat had no interest in going outside to begin with. Despite her best efforts, I kept my cat inside. I moved shortly after into my parent’s home, and since they have a Lhasa Apso, they gave my cat free run of their gigantic, apartment-like basement. Since we used to live down there ourselves, it had carpet, rooms, heat, etc. I spent most of my time with her downstairs to make sure she was okay. She did great, and never once showed any interest in the doors leading outside or upstairs. I “moved” once again (but it was supposed to be a long vacation - and instead of burdening my parents with looking after my cat, I packed her up and took her with me. She sat in her carrier with me on the plane, in her very own seat, and she handled the trip better than I did). In the apartment that became our unexpected home, again, no interest in the front door. She spent much time in front of the big patio doors, and loves staring out windows, but she never meowed to go out, ever.
Shortly after I got married, my husband and I noticed she did spend a lot of time whining outside out bedroom door at night when we went to sleep. (No kitties in the bedroom at night, or hubby can’t sleep!) So, we had room in our apartment and our hearts for another cat, and we went down to the shelter and rescued another female cat, a couple months younger than our first cat.
Cat number two was an eight month old stray cat (wth a LOT of health problems, poor thing), nobody knew where she was from or what she’d been through, but she was a mess. She had been picked up wandering in an alley and brought to the shelter. She also showed no interest at all in being outside. We don’t know if it was her previous experiences with being outside that turned her off to it, or if she just really, really liked being warm, fed, and loved. I would say option two.
Getting a second cat also shut the first cat up - no more whining at night. We moved to a smaller apartment, and they still have no problems. They discovered new circuits to run, new doors to open - but never the front door.
If we left the door open, they would be curious and sniff around, but they are mostly scared of the outdoors. Sometimes when we come home we’ll see a little nose in the door, but they make it pretty obvious that they are just greeting us, and don’t want to go out. They skip around and dance backward when they see it’s us. If cats could cheer, they would. When the door is closed, they don’t care about it. What’s kind of interesting is that every other door in the apartment, they whine and cry and scratch and beat down until we open it. The door to outside, however, is silently understood to be verboten. Neither cat has any problem with this.
For the record, our cats are both about two years old, and are very healthy and very happy. Mr. Stasaeon takes time out of his day to play around with them (he’s just a big kid, after all), we give them lots of toys (they have so many now we have a “toybox” - they know to go over to this box and haul out their toys when they feel like playing). One of them is very petite, slim and trim, and moderately active, the other is bigger, getting a little chubby, but she is still very active. We changed her diet a bit and she’s losing her big tummy. That’s something to watch for, don’t let them get too lazy, or overfed, or they’ll get chubby. I honestly don’t know how that one got chubby, since she is the most active cat I have ever known. She bounces off the walls, and is the one who begs us to be played with, like a hyperative puppy. Either way, we monitor their food bowls closer now, so we know who’s eating what and how much. Overweight cats shouldn’t be a problem if you carefully monitor their food intake, and make sure they have toys to exercise with. Or even do like hubby does, get down on your hands and knees and play with them yourself - we chase them around, they chase us back, and then they roll around for tummy rubs. They play with each other often, too. I’m a big fan of having two animals, to keep each other from being lonely when we’re out, and to have buddy to play with for extra exercise, if that’s at all possible. It’s understandable if you don’t have the time, money, room, etc., of course. But I would highly recommend it if it’s possible. Besides, what’s better than a warm, purring cat in your lap? Why, two warm, purring cats in your lap!
I would say interest in outdoors varies from cat to cat, of course, as others have said, but I think if you are firm and constant in your training and commands, they will learn. And they won’t be unhappy if you keep them entertained, or them plenty of ways to be entertained.
Whew, okay, sorry for rambling on. I don’t have children yet, you see, so these are my only “babies”! Good luck with whatever you do. Fuzzy companions are fun and worth it.