If you have the right kind of door to the balcony, you could put in a cat door so and have it held open during the day. You could put an enclosed area for them right next to the balcony. I'd use a puppy playpen, something like this:
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With linoleum flooring underneath it and cardboard zip tied to the bottom parts of the panels. That way you can put dirt/pine shavings down for them, and give them full balcony access. If you put bird netting around the balcony so that they can't fly away, they can be on it without supervision.