out for their first explore when they’re hungry
Yep, I would also do it early in the morning, firstly because if you end up having to look for them you won’t be doing it in the dark. And secondly, because cats feel considerably braver in the dark than in daylight, so are much more likely to wonder further.
I would also not let them out together as they are more likely to egg each other on doing little sprints to overtake each other, and therefore go further than they would have done on their own.
Just 10 or 12 feet from the back door for a couple of minutes is enough for the first time imo, enough to sniff around, get to know the area round the back door, and figure the way back into the house. Build it up a bit at a time.
You’re very unlikely to lose them imo, cats aren’t stupid and they will only go as far as they feel they can whilst still remembering the way home. The biggest problem is when they get really spooked up and they take off forcing them to go deep into territories which they don’t know. For that reason I would be worried about next door dog, if he suddenly goes ballistic they could take off. Try doing it at a time when the dog isn’t around.