I'd guess it has less to do with the distances involved and more that fact that the sound waves had to pass from one medium to another. Water is actually a better conductor of sound waves than air if I remember correctly, being denser. However it may not be effective at transferring much of that sound energy to air, so only a relatively small portion of the sound energy actually hits your ear.

Water conducts sound. Actually 4 times faster than air.

However the interface between air and water presents an impedance mismatch and only a little of the energy can cross from your daughters mouth into the water, though if you are underwater too, you can hear it.

If your in the air, the mismatch happens twice. Once from, your daughters mouth to the water, and again from the surface of the water to the atmosphere.

It becomes very hard to detect, though not impossible in very quiet surroundings and with high gain audio equipment.