Short answer: No, WiFi signaling (a form of radio waves) itself is not hazardous to anyone's health and has no scientific connection to causing cancer.
Long answer: An understanding of light and radiation is important to understanding how there are different categories of light and radiation which have different characteristics such as energy level and potential danger posed to organisms' health. In a very short summary, light travels in different sizes or "wavelengths" that relate to their varying amplitudes and frequencies. For example, ultraviolet or "UV" rays have shorter wavelengths and therefore a higher frequencies than visible light, when considering waves of equal energy.
Notice how much longer the wavelength of "radio waves" are in comparison to "Ultraviolet waves" or "X Rays" which are harmful to humans upon extended exposure. UV rays cause cancer because the energy that these waves have allow them to change and damage the structure of DNA in your cells so that they don't function normally (specifically, the rays break bonds between the nucleic acid bases that are the units that compose your DNA and then cause them to bond incorrectly/not how they originally were bonded, which causes your genes to be translated into incorrect proteins or to terminate protein synthesis. Proteins are integral to cellular function.) Certainly extreme exposure to radioactivity can cause harm across different types of waves, but one must consider how the waves are "delivered." You can cause damage to someone theoretically with microwaves if you put them in a theoretical microwave and subjected them to a lot of DIRECT microwaves which heats them up.
Wifi radio waves are very "weak," so-to-speak, in the form of how they're used by electronics as we know them. The following is quoted from this source (Wi-Fi: are there any health risks?) to explain why wifi is not harmful:
"the intensity of a Wi-Fi signal is around is 100,000 times less than a microwave oven. The oven is a targeted device that operates at very high voltages and short distances. Wi-Fi routers operate at very low voltages, broadcast in all directions, and are used at relatively long distances.
Since radio waves follow the inverse square law – like light, sound and gravity – then each time you double the distance, you get only a quarter of the energy. In other words, the signal strength falls off very rapidly. At normal operating distances, Wi-Fi's intensity is generally so low that it's not worth worrying about: it's just part of the "smog" that is generated by radio and TV signals, AC mains wiring, the motors in home appliances, and the universe in general. (As my colleague Charles Arthur once pointed out here, the wavelength of Wi-Fi signals is the same as the cosmic background radiation: 12cm. If you're worried, don't go outside.)non-ionising wavelengths that are longer than light tend not to be dangerous. These include infra-red rays, microwaves and radio waves. At 2.45GHz, Wi-Fi comes in the microwave band along with baby monitors and mobile phones.
Now, it's certainly possible to do dangerous things with radiation, even if it's just focusing the sun's rays with paraboloid mirrors to set Roman fleets on fire (not that there's much call for that). It's also possible to use a high-pressure water jetto cut through steel, but that doesn't mean you'll die from taking a bath or standing under a fountain.
Of course, it does make sense to minimise risk, as long as you concentrate on the biggest risks, not the trivial ones. If you want to do that, the mobile phone must be the first thing to go. In use, the phone is held close to the brain, while the Wi-Fi router may well be in another room (inverse square law). It has been estimated that you get a bigger dose of microwaves from one 20-minute phone call than from a year's Wi-Fi.
Twenty laptops and two routers is roughly equivalent to one mobile phone.
The World Health Organisation, which has examined the topic in depth, says: "In the area of biological effects and medical applications of non-ionizing radiation approximately 25,000 articles have been published over the past 30 years. Despite the feeling of some people that more research needs to be done, scientific knowledge in this area is now more extensive than for most chemicals. Based on a recent in-depth review of the scientific literature, the WHO concluded that current evidence does not confirm the existence of any health consequences from exposure to low-level electromagnetic fields. However, some gaps in knowledge about biological effects exist and need further research."
Side Note: So far I've been talking about wifi signaling as itself, a functional form of the electromagnetic spectrum. However, one could be a bit of a musing, semantic troll and claim that wifi can be harmful to people...if you use it to connect to internet and harm yourself or others with the information you find on the web. Or perhaps you could cause harm by not living an active lifestyle and just being a unhealthy, sedentary web surfer all day as enabled by your wifi connection.
I hope this answer helps!
