Nothing wrong with staying inside. Even a cat that has gone outside their whole life can live inside without any protests while those who have lived their entire lives indoors can be introduced to outdoor lives. Obviously these things are easier to teach to a cat when they are young, as old cats tend to be autistic and hate everything new (including changing homes), but it's not like it's impossible or anything.
The only thing that really changes is that a cat can be more independent outside. They will go where they want, amuse themselves and get food on their own. You have to provide a cat that is indoors all of this on your own, so you need to actually play with the cats so they don't get lazy and weak. You tend to create a different bond just because you are around each other all the time and I do prefer it to a life with an outdoor cat.
As for leaving cats on their own, they sleep most of the day, kittens or adults. My cat goes to sleep when I leave and wakes up when I come back so we can do stuff together. Thing with kittens is that they're curious and stupid so they tend to get into trouble if you don't watch over them, but luckily they grow out of that fast as long as you get one at the age they are supposed to be given out (4 months).
Making your house kitten proof enough is a good thing. Block any small holes they can crawl into, remove things where they could get stuck, be it their body or something like a claw. Try to make sure they can't climb high on their own, avoid curtains they can reach. If you think something bad could happen with something, trust your instincts and be safe rather than sorry. Also get into a habit of checking that pile of blankets before sitting on it in case there's a kitten sleeping in there.
