the babies need brackish water to survive. i've had 5 pregnant shrimps in my 6 gallon (only freshwater) about 2 months ago, and i've yet to see one baby shrimp.
you can try starting a 2.5 gallon brackish tank as an experiment in raising the fry... let us know if it works out...
"Larvae hatch into a protozoea stage that has no mature appendages, small enough to be confused with plankton, then pass through metamorphic stages, becoming more shrimp-like."