What could dark yellow urine in babies mean?

Concentrated urine. Concentrated urine means that you need to hydrate more. In the baby, it means you should try to get them to take more fluids. Babies should only drink breast milk or formula, 24-32 ounces a day, if under 6 months. Up to 6 ounces of juice can be added at 6 months. That usually takes care of fluid requirements.
Can be normal. Dark yellow urine can be normal, it can mean they are breaking down red blood cells and producing more bilirubin than usual, if can mean they are a bit on the dehydrated side of things, or it can mean they ate a bright yellow big bird piece of cake. I would worry more about urine that is tea or cola colored than just dark yellow.