How do you help a childs stress levels in a marriage separation situation?

My daughter and her SO split up about a year ago. It was an amenable breakup and both parties are still communicating and taking care of my 7 year old grandaugher. The only problem I see is that it seems to be taking a toll on her mental well being.

During the week they take turns taking care of her every other day and they compromise on the weekend depending on each others schedule. But, it just seems like switching parents every day is taking a toll on her. She seems to be emotionally stressed out, prone to tears for the slightest reason and, while a generally happy child, she seems to mope for no obvious reason.

It could be she is just tired. I know her mom enforces a bedtime of 8:30 but her dad does not seem to be able to set his schedule to do the same. It is not unusual for her to go to be after 10 pm. She is also three weeks into the school year and has been late four times, each time it has been after staying with her dad. They have spoken about this but he does not feel that 2nd grade is that big of a deal.

My daughter is fretting and not sure what to do. She does not want to deny him his daughter but does want her to be happy.

My suggestion is for her to keep my granddaugher Sunday night through Friday and for him to have her all weekend.

Any thoughts, particularly from those of you who may have been in similar situations.