Of course there is some danger when the cats can go out of the balcony. Especially the younger one who will be more active could easily fall off. And cats can hurt themselves and even be killed when they drop from a height like that.

A friend of ours moved to a different house one day. The very first day, kitty went on the balcony in the second floor and walked right over the edge I am glad to say nothing serious happened- it was mainly a shock.

I have a netting all around our balcony. But this may depend on whether the house owners allow it.