Should I let them out?

At the beginning of Nov my bf and I will be moving into the apartment right next to ours as it has a balcony and aircon. The unit we're in now doesn't have these things. We've always lived in a one bedroom, which means that Nermal and Lily have always been indoor cats. I was just wondering how safe it would be to let them sit out on the balcony while we're at home of course?? I have heard of indoor cats suddenly being let outside, and I am a bit worried as I've heard that these cats have run away or gone missing.
I know that being indoor cats all their lives and then suddenly being able to go outside would be a huge change for them.My instincts tell me that I should keep things the way they've always been - I don't want to lose my babies.
Any advice would be greatly appreciated.

I would say it depends on the balcony - is it high, can they jump off, can you add some fencing/screening/chicken wire to enclose it for them? If you are planning to use the balcony a lot yourselves then I would try and make it safe for the cats too, as otherwise you will have a very difficult time with them and be forced to keep the doors closed all the time. You can also get them used to it by taking them outside on leads and harnesses.

I would enclose the balcony. A cat after a bird will just jump after it no matter how high the balcony is, and so can fall and die or be severely injured. It can also be lost or stolen.

If you can't enclose the balcony safely, then get a portable cat enclosure (like a tent) and let them sit there for a few hrs. But I would NOT let them loose on a balcony.

Thanks guys, for the advice.

It's helped a lot. We stay in a complex on the third floor so the baclony is pretty high.We've decided to put up some type of mesh so that they can also enjoy the outdoors.