Hi there, my boy is FIV+ so has to be an indoor cat. He was picked up as a stray when he was 2, so had been used to the big wide world, but he's been kept now as an indoor cat for 6 years and he's absolutely fine with it.
It will almost certainly be easier with kittens, as they will not have been outside before, and will not be aware of what's out there!
As everyone else has said, you do need to make sure they have plenty of stimulation, so lots of playing when you are at home, and toys they can play with on their own when you're out, so they don't get bored.