I've had all four of those breeds at one time or another over the years (still have Buff Orpingtons and Black Australorps, and have seriously conisdered getting some more Sussex) and there isn't a lot to separate them. The three listed in the title all have approximately the same lay rate at 3-4 eggs per hen per week (Australorps are better at 5, occasionally 6 eggs per hen per week). All three of them have approximately the same hardiness (all are very cold hardy, Australorps as well and they are more heat hardy than the other three breeds). Which of the three listed in your title is the most beautiful is a subjective matter (I personally think the BOs are the prettiest), and only you can decide which is your preference in the beauty department. That leaves temperament, and how good they are with children. While all three breeds are generally docile, in my experience Orpingtons and Sussex have a slight advantage here over Barred Rocks. I have on infrequent occasions had an aggressive Barred Rock that I had to cull from the flock (this is definitely the exception and not the rule). Orpingtons and Sussex are both very friendly, calm, and gentle (I've never had an aggressive bird of either breed), and my children made lap pets of both breeds. I personally think you've made the very best decision with the Black Australorps, but either Orpingtons or Sussex would be an excellent choice as well. Because everything else (lay rate, temperament) is so equal between the Orpingtons and Sussex, I would suggest getting the breed that you think is the nicest looking. Whichever breed you decide to get, good luck with your flock.