Thanks for using DoctorSpring for posting your query. I have gone through your question in detail and I can understand what you are going through.
I understand that you are trying to bring up your niece (6yrs) who is having parents with troubled relations and the child has started presenting behavior problem. The behavior problems that you have mentioned like not eating regular food, staying alone and not mixing with friends, and reduced academic performance, can occur in the child who stays away from parents.
Initial 10 years are very crucial years and the child should be with the parents ideally. This would be the best thing for your niece. However if it is not possible then there are certain measures that you need to take.
6 yr old child is amenable to behavior modifications. You need to identify the problem areas one after another and take it up one after another. Like to start with, eating her regular meal. If she has her meal she should be getting a "star" and if she doesn't then she should not be given a "star" during that day. If she collects 5 starts then she should be allowed to do things which she likes like may be going to a movie or play zone or eating a burger etc. This is called conditioning. This has to go on for a few months.
In case of temper tantrums the best way would be "time out". In this process if she doesn't listen to you, she should be debarred from what she is doing for next 10 min and kept in a safe room with out any entertainment where she would get bored. After 10 min, she should be released with a warning that if this recurs then the same would be repeated.
Its good to shower love, but there should be firm boundaries for her behavior.
You should never punish her or beat her as this will increase the problem.
Hope this helps,
If you any further queries, I would be glad to help you further.
Dr. Srikanth Reddy MD