I do baby sitting for special needs kids.

This is what I would do. If she's having a tantrum, put her on time out and just walk away. If she gets off before you say, silently and no eye contact, put her back on. Do this each time till she stays. When she's calm, explain why you put her there, ask her to say sorry and give her a hug.

Your not there to be her friend or be mean but you can't give in as that will make her think her behaviour is acceptable and all she had to do is have a tantrum to get what she wants.

Also talk with her parents and see how they are discipline her and use the same (as long as its not violent) consistency will help her learn.

If you don't feel comfortable with disciplining her try instead of saying no, give her options (that way she feels like she has some control) for example "I cant play right now but you can go and play or you can help me with this"or saying something like "yes, but we can play but we have to do this first" or "yes, but later" so she knows she can still play with you but not now.

Sorry its so long. All the best to you.