Mrs. Milo,
My two older boys were in bunk beds for a couple of years. They were 7 and almost 3 when we did this. It worked out great. Now we have them split up, the older one has his own room and the two younger boys have a room. Once the baby is older we will put the bunks beds back together.
I think I remember reading that the American Pediatric Association recommended 6 and over for bunk beds.
I guess it would really depend on if your DGD's get up a night, my middle one still does, are they afraid of heights or if they move around alot. I was nervous at the beginning the oldest would fall over the side. But I am a worrier.
I hope this helps.
Sharon