Worried that a child is a victim of abuse, perhaps sexual.

Hi, sorry if this is not in the right section.

My mother child-minds an 8 year old girl and we have become concerned about her behaviour. I have been doing some research online but can only find information on how to tell if your OWN child is being abused which involves asking the child questions which we would not really like to do at this stage incase we are wrong as she maybe has a behavior disorder instead. We have known her family for many years now and my mother also looked after her elder sister for several years without any indication of anything sinister. This is why we sometimes feel like we are jumping to conclusions, but at the same time we know we should not put off taking action incase something terrible is happening to the poor child. Here are some of the things we find abnormal:

She masturbates (vigorously and very often), sometimes with her shoe. She does this when she thinks an adult is not present however we have seen her do this in front of other children.
From the age of about 2-5 she suffered from unexplained hair loss and was almost bald at certain stages. She has hair now, but it is very thin.
She is not doing well at school and it is sad that she is about the same level of intelligence as my niece who is 5 years old. She also gets into trouble at school regularly.
We are shocked and upset everyday by her mean spirited attitude and how she tries to get other children into trouble via blackmail. Also, she is very nasty to my mother sometimes, calls her names etc and tells her parents that my mother hits her which has never happened. At other times however she will act normally towards her.
We have noticed that when someone picks her up to go home she will always ask to go to the toilet before leaving, as if she is trying to prolong her stay in our house as long as possible.
She also steals a lot from school and from other children, items as valuable as a Nintendo DS and mobile phones. (I am not sure if abuse would cause these sort of behaviors in a child but I feel it is worth mentioning.)
She lies about a lot of things that are not unimportant e.g. she has told my mother that her parent is in hospital or that no one is coming to pick her up that day.

Do these behaviors suggest abuse and what should we do about it?