The answer lies in the size of the pest compared with people. A flea is millions of times smaller than a human, and therefore requires only a tiny amount of a toxic material to cause its death. Following the application the water evaporates, and the active ingredient of the pesticide binds tightly to the sprayed surface, virtually eliminating exposure to you or your children who may contact that surface once it is dry .
Weed control is different, since plants have metabolisms far different from animals.
For example, Roundup herbicide has an acute toxicity that actually is about one half the toxicity of table salt. Now, please don't think that this means it is okay to drink Roundup, but with standard precautions and the use of Roundup as outlined on the Product Label that comes with it, there should be no hazard created for you. Roundup is designed to affect a plant, and at the doses adequate to do so normal human exposure is well below a risk level. However, used correctly they can provide benefits for us in eliminating problems, and the risk from the toxic substance is kept extremely low. Renowned researchers in this field of medicine, such as Dr. Bruce Ames from the University of California, or Dr. Edith Efram, another well known expert on cancer and its causes, believe that pesticides are a negligible concern in this area.