Well, what is she normally up to during the day? Can dad just take the lead on letting her pop back and forth between the labor and her normal activities as she wishes? Do you have any friends or family that would be willing to take her for that time period, or that you would be comfortable with in your home caring for her?
My dd is 10 now, but has been obsessed with where babies come from and human biology since she was a toddler. She says she wants to be my doula for any future births. =D I was preg earlier this year and had explained that dh was basically going to by my midwife. Ds has suggested he can help with preparing for the birth, making meals, baby care, etc (basically he's more than happy to do anything that doesn't involve the actual birth, if we will kindly exempt him from the more intimate stuff...but, really, he's witnessed a couple births now and is really into them so in the moment he's going to change his mind. lol) Maybe talk to your dd about what she can do to help? If she feels like she's helping bring her little brother or sister into the world, she may be more into it?