You are doing all the right things!
Thank goodness she has the stability of grandma/grandpa during this very stressful time.
Her behavior for her age is completely appropriate..She is experiencing adjustment issues (as are everyone in the family!) This will improve as she becomes accustomed to the routine.
I would go to the store with her, buy a colourful, fun calendar, and help her mark the days she is with mommy, daddy, and you. She may appreciate being able to see when she is moving.
At each home she stays/visits, have an assortment of her favourite books, clothes, toys etc. so she feels comfortable when she moves in: she feels like she's "home".
Have pictures of mommy and daddy at your house visable (on the fridge etc) and pictures of you and grandpa also visable at her mom or dad's place.
Allow her to phone mom/dad each night at bedtime if possible, or teach her to type out an email. She'll get the hang of it!
Last but not least, validate her feelings. It's normal to miss mommy and daddy. It's okay to be upset about the situation. Reassure her that none of this is her fault, or had anything to do with her behavior/personality etc. Kids often feel they are the cause of the breakup.
If none of this works after a few weeks, suggest to mom and dad that they do a bit of short-term family counselling in a children's mental health centre to address her adjustment needs. It will pay off in the long term.
I hope this has been helpful.
Mary Lynn Trotter MSW RSW