Our two bundles of fluff are five, and we shut them in at night. Free to roam during the day (and yes, they do roam a long way - particularly big boy cat ) but we get so many foxes around here, and cars bomb along our road in the early hours, that I just don't feel safe letting them out. Also according to the RSPB it is better for potential prey if cats are kept indoors at dawn/dusk - which is another excuse I use for night time cuddles!
Yes, they've been neutered/spayed. And they have a micro-chip catflap so they could simply come and go as they please like they do in the daytime. My concern probably is what you've mentioned - their lack of sense! They are certainly very adventurous but I had read somewhere that kittens are more likely to wander further than older, more wary, cats. It's good to know that I'm not necessarily being precious about them!
I wouldn't be letting them out at night yet. They're still small and not that sensible. Certainly as Winter is coming I would have thought they wouldn't be that keen on going out at night anyway and I would wait until the Spring.
However I would only let them out at night even then if you have a cat flap so that they can come back in. If you don't have a cat flap then I would never let them out at night. Partly because it's a long time to be out and partly because they are in more danger at night (road accidents/foxes - not that foxes are generally a threat to cats but some particulalry vicious individuals can be).
We have gorgeous 6 1/2 month old kittens. They've been allowed out (while we're home) since they had their jabs and last week we had a cat flap fitted so they can come and go as they please (and don't they look happy about that! ). But so far we've balked at allowing them out at night. I'm not sure why - just a feeling that they are too young. Are we being ridiculous and a little pfb?! Or should we be keeping them in at night til they're older? Thanks