Tigger by informing the Police you are not condeming these dogs or their owner to anything other than an investigation.
The investigation could prove nothing, but alternatively you could be highlighting some issues which could ultimatly save people from an attack from these dogs.
I would suggest a anonymous call to the local Police, cause at the end of the day you'll probably feel less guilt if these dogs are removed or even deatroyed than you would if they attacked a child and you did nothing.
If this person is a responsible dog owner they will welcome any investigation and if they are not they do not deserve to own a dog.
B. Lucky
