By cupful I assume you mean that the Miralax bottle has a cap and this is what you are calling a "cup" and that one cup = one dose as prescribed by your doctor.
At the time he was 10 so I'm uncertain how well this applies, but at one point my son was on 6 doses of Miralax a day (3 x 2 doses) plus 2 doses a day of Benefiber.
At other times he was other combinations of various stool softeners and laxatives.
You might want to talk to your doctor about a trial of mineral oil. Our current ped GI prefers this and I have to say that it works really well. This would not be for chronic daily use however. And be forewarned, because it isn't absorbed the oil comes out with the stool. It does absorb bile which turns the oil a brilliant shade of orange.
As others have said, be sure she's getting enough water.
I assume you are also doing dietary things to help. Apple juice, for example, is a good substitute for prune juice if she doesn't like that. Pears with the skin on - either cooked or fresh - are great as are cherries.
Do you have a toileting routine set up where she sits on the toilet for 5 - 10 minutes starting about 20 minutes after she eats? Even if she doesn't go it's helpful for her to learn the routine that will support her body's natural rhythm - peristalsis (the muscular contractions that move food through the gut) begins when we eat and stimulate the body to "go" roughly 20 minutes after we eat.
Hope she gets feeling better soon.
