I would report it as it is ILLEGAL under Dangerous Dogs Act 1991. Of course, we have heard of smaller dogs attacking babies, but these pitbulls are powerful dogs.
I remember my son being chased by a family dog, not even a pitbull. The dog was let out together with the owner's children on the same route to school. This bundle of energy just chased my son, along a road with parents driving their kids to school. I was in tears and totally helpless.
In another incident, I was in Hampstead Heath, a huge dog jumped on me, didn't attack me, but that really gave me a fright. I had my period and I was sure the dog smelt that.
When my friend and I were walking in a park, another small dog pounced all over me, leaving footprints and my friend suffered from the indignity of being sniffed at the bottom. I was annoyed and the owner said, "It just wanted to say hello. Judging from your face, you want to kill it!" I asked you. We were just walkers trying to take in fresh air in the park and yet being messed about by the dog and then the owner had a go at us! We didn't say anything. Dogs out being walked should be on leash. Or at least, respond to owners' instruction.
One day, I saw a teenager being pulled by 2 huge dogs outside my house. She was dragged by the dogs! I'm not against dogs. They provide companionship to a lot of people. But once a dog has attacked animals or children, it should be put down. Once it has the taste and biting experience, there is no way of knowing what the next trigger can be!
Report it as these people are doing something illegal and usually when they do things like this, there is something dodgy about their character - show a disregard for law, and safety for others. I remember seeing photographs of policemen being mauled by big pitbulls in South London. No laughing matter!