When you place a seashell up to your ear, you are likely not hearing the ocean. Science suggest that you are actually hearing sound that is concentrated resonate noise from the surroundings, such as the echo of the noise in the air around you, also referred to as ambient noise.
The larger seashell captures the ambient noise, which then resonates inside the shell due to its internal surface material and angles.
Of course, there certainly is no harm in letting a child or an adult for that matter, believe it is the sound of the ocean, as it certainly is a beautiful sound. I personally choose to believe that I am hearing the sound of the ocean, and perhaps maybe the blood rushing about in my body, feeding me life.
FYI you can also accomplish this with a large ceramic coffee cup to the ear.