As everyone else already pointed out, there is no reason to be concerned.
Everything in the known universe is subjected to radiation of some variety. Electromagnetic radiation can be broadly categorized into ionizing and non-ionizing. "Ionizing" radiation means it has sufficient energy to liberate electrons from their orbit while non-ionizing doesn't. Neither category is considered particularly beneficial to human health, but life of all variety nevertheless evolved and continues to thrive in its presence. Evolution would not occur without it, and life would not be possible in its absence.
Of the two, ionizing radiation has the ability to alter human and plant cell reproduction and growth due to the energy it carries. Examples of ionizing radiation are x-rays, gamma rays, radiation from the sky (cosmic rays), even radiation from the ground (radon gas - so forget about living in a cave). It can harm you, if you are exposed to it in sufficient quantity and duration, but negligibly small exposures are also of negligible concern.
Non-ionizing radiation does not have sufficient energy to alter cell reproduction and growth. Examples of non-ionizing radiation are radio signals, radiation from power lines, and the Wi-Fi you are concerned about. It is all around us and there is no escaping it either, as long as people do the things that generate electromagnetic energy enabling them to work in a civilized society.
There is no single distinct line separating these two categories but the spectrum of visible light can be considered as such, broad as it is. At its two extremes there is ultraviolet - closer to the ionizing category, and infrared - closer to the non-ionizing category. Prolonged exposure to ultraviolet light isn't a good idea; it can promote skin cancer for example, but your layer of human skin prevents it from reaching internal organs. Prolonged exposure to infrared light just makes you too hot. Infrared space heaters are useful to heat people working outdoors because the radiation isn't blown away by the wind. If you have an electric infrared space heater you know how much power it uses when you pay your electric bill.
Not only is your AirPort's radiation well within the non-ionizing region, its power is orders of magnitude less than your infrared space heater. It can't possibly harm you or your children, even if you were to put it under you pillow every night.
Cats are another story: Airport Extreme Safe for my Cat to Sleep on?
