Hi, welcome to Purrs
As well as being neutered, make sure that they are de-flea'ed and wormed regularly as they are more likely to pick up these things being outdoors. Also, make sure that they are microchipped. I don't advocate collars as they can get caught, even the so-say 'safety' collars but some others on here might disagree with me. Also, best to keep them in at night. Um .. what else - oh yeah, regularly check them for any signs of fighting, and keep and eye on any bites or such that you might find as they can become septic. Oh, and make sure that they have had all their jabs as, again, they are much more likely to contract cat flu, leukaemia etc if they go outdoors.
Edit: And also, when letting them out for the first few times, do it when they are likely to be hungry, so that they will not only come back fairly soon, but also get used to coming back when they are ready for a meal.
Hope your babes enjoy their new world!