Babysitting two difficult kids. Help?!

Later tomorrow evening I am babysitting two very difficult kids. Although I have done this before, they are really hard to handle. First, both of the kids I am babysitting are brothers, one is 8 and one is 6. They both hate each other. They fight often, which starts as a funny wrestling match, then turns into one kid biting, screaming, and pulling at hair. The mother tells me to put the kids to bed at 8:30. However, the kids are so energetic they refuse to go to sleep. The problem is, the kids like me, and will not go to bed because they are so eager to play with me. I have had days when they stayed awake until 12:00 AM because they are so hyper. I have tried most options to contain them. I have tried bribing them, which never works, I have threatened to call their mother, I have fake-called their mother, and tried to watch TV with them, but nothing works. The eight year old likes to play on his ipad, the 6 year old likes to play with me, but the eight year old gets sad if he plays on his ipad for 3 hours and is bummed that I do not hang out with him. When I say to the 6 year old I am going to hang out with the 8 year old, he gets angry and freaks out. They always ask for gummy bears, and I usually give it to them to keep them quiet. However, the sugar hypes them up, keeping them awake for 2 more hours.

I feel really bad when the mother comes back at 11:00 PM and sees that her children are not asleep yet. I need a way to make them tired so they go to bed.
Please help,thanks

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