Dehydration is certainly a concern for little ones although your DD is older so that helps. I agree nursing as much as possible is best, since breastmilk has electrolytes like pediasure in addition to all the other good stuff. Something to try would be to drink lots of water yourself to keep very well hydrated so that their will be plenty of water in your breastmilk. Short frequent feedings will mean that she's also getting more dilute foremilk instead of rich hindmilk, which might be easier on her tummy. So even if she only nurses a bit, it's worth it. Also, even if the breastmilk, pediasure, or water is mostly coming back up her body is getting some of it so keep trying even if it takes squirting in her mouth as macgiver mentioned.
But if you notice any of the signs of dehydration mentioned above, especially a sunken soft spot or skin that doesn't bounce back it is important to call your Dr. right away. With small bodies dehydration happens quickly, so while that's a scary thought as a PP mentioned it is treatable with IV fluids, which also sound scarier than they are. I hope she feels better soon!