Oh god, how horrible for you to hear that- it'd break my heart to hear that coming through the walls!
You can phone Childline anonymously (0800 1111) and even contact Social Services by email
http://www.rbwm.gov.u...m/?F.Name=h-pf-jkr7aw
Have you thought of talking to the wife of the couple? Not sure how you'd go about it, but maybe ask how they're getting on with the toilet training or something? See if she tells you anything?
I know it's easy for us to say you must do something, we're not in that situation, but I know i couldn't live with myself if on the slim chance the little girl gets hurt.
Good luck, let us know how you go on?