How is it possible to prove that someone is an unfit parent?

If you have serious concerns, contact the Child Protective Services agency in your area. They will investigate the home living situation of the children involved.

CPS is who you need to contact but from what I see all to often the parents paint a rosy picture and CPS leaves them alone thinking somebody just called for spite.
Make sure you tell them all of your concerns so they know it is important to do more than have a simple talk with the family.