I am so very sorry for the loss of your son. Samantha's parents seem to be ignorant about the whole situation. I know that she is only 8 years old, but it seems like loosing weight is a decision she will need to make on her own as her parents seem to be ignoring the situation. Have you ever seen that show "My 600lb Life"? Maybe she could watch that with you?
There is so much controversy about having morbidly obese children taken from their parents. I think letting your child become obese is a form of child abuse. A boy from my high school died from obesity in his parents home. He was too big to leave the house so the parents were clearly buying the food for him. He was so large that they had to cut him out of his home when he passed. It made me really think about what was going on with those parents. I really think that they were giving him the food because they felt bad for him. Here is an article published by CDC that might help
Healthy Weight: Tips for Parents | DNPAO | CDC
In essence because she is so young...her parents really need to get on board with the weight loss. I think if the problem continues you should call social services. Could you talk to her teacher or the school? Maybe they can make the call so you are not stuck in the middle? Also, hiring a nutrionist might help too. How tall is she if you don't mind me asking?