The numbing rumor is unsubstantiated.
Most baby shampoo's use sodium trideceth sulfate and/or a non-ionic surfactant. Surfactants are the chemicals that allow water to surround non-charged particles such as dirt and "dissolve" them into solution [think detergents and soaps]. Sodium trideceth sulfate irritates less because it is not strong enough to strip off the protective layers of the eyes and skin. The exposure of the underlying tissue to the environment leads to the irritation and resulting pain.
A similar comparison can be made between regular soap and dishwasher detergent. Compare showering with soap vs. dish detergent. The red irritation you observe from using dishwater detergent is, in part, a result of the stronger surfactant used in dishwasher detergent [please do not do this].
We could attack this argument in another way as well: If the lack of irritation was due to a numbing agent, the exposed individual would be expected to exhibit red irritated eyes with no associated pain, a.k.a. numbed pain. What you actually find is no red irritated eyes and no pain, a.k.a. no need to numb pain.
Interestingly, you cannot use baby shampoo on dogs since it IS strong enough to remove enough of their skins oil to produce irritation and pain. Animal testing = BAD.
Please also consider that some individuals may suffer an allergic reaction to any ingredient in any product. There are indeed babies and adults who react negatively to generic baby lotion for this reason. So always test new products for side effects before slathering them on!
Severe allergic reaction = BAD
For a breakdown of each of the chemicals in J&J's baby shampoo check
this out.
