It sounds like her parents are doing a really good job if she is healthy!
I too have a child with DS. It is very difficult to get them to eat, so I understand. Apparently this is a common problem. But that's wonderful your grandchild is breastfeeding! There is nothing better. I think if I had been able to breastfeed, my child would have been better off. Is she eating any foods yet?
Does she suffer from GERD or acid reflux?
One main problem in feeding seems to be the low tone in their mouths and tongues. Does her tongue protrude a lot? Feeding specialists can work with her low tone so that the mouth responds better, because every time my child goes to eat food, his tongue shoves it out. This has gotten better with time and with OT.
You have to identify first what her problems are. For example, if she starts eating regular food, and she has problems with constipation, then she will stop eating because she will feel full. Kids with DS have chronic constipation because their intestines are too small and its harder for them to push the poop through. And the stomach could be digesting food slower, because of low tone in the stomach. It could be that she doesn't want to eat more because of this. My child has gastroparesis because of his stomach not pushing the food out. This causes him to not want to eat.
You can try boosting caloric intake by adding baby cereal to her milk in a bottle. Also you might be aware that kids with DS can develop celiac disease, which causes more issues, so I would stay away from wheat and do a gluten free diet, which is easy right now. Just give rice cereal instead of wheat or oat. One thing I have heard too is that during the first year of life, babies with DS are underweight. Then after the first year, they begin to gain momentum, and start gaining weight, sometimes too much. Then you have to be careful, because they can become obese.
What has helped me to understand why my baby has all these problems is that kids with DS have super low muscle tone in practically everywhere in their body. Stomach, mouth, legs, arms, feet, etc. One adult with DS described it as feeling as if you have 5-10 lbs of weights strapped to each arm and leg and then trying to walk that way. I have researched so much, but it has helped me to be able to take care of my child better.
One thing I want to say to encourage; my son is the most joyous thing there is. Don't worry about the Down Syndrome itself: your granddaughter will delight you in ways you've never imagined. It may take them awhile to do something, but once they do, its a true celebration of a magnitude that I never experienced with my other children. They have to work so much harder to get there, but once they do get there, there are no words for how good it feels.
If your daughter wants more info she is more than welcome to contact me here. I would be happy to help her as I wish I had more help when I went through this a year ago.