Only you know what exactly happened with that bite. If it was one of my cousin's kids, I can see how a Rottweiler or any other animal or person would need to protect itself from a four year old. Those children will never, even as adults ever be allowed near any of my pets
The part that bothers me is that you have doubt. If you are sure in your mind that you have control of the dog and it will never harm anyone again, then keep doing what you are doing. If you can't be sure of that and keep doubting your decision, wll then reevaluate it. It's a rough position to be in.