Every child in the history of the world has been exposed to electromagnetic radiation. As I mentioned, it includes ~visible light~. Unless you keep your child in an entirely dark cave for their entire life, they’ll get a whole bunch of EMF.
That being said, obviously there are types of harmful electromagnetic waves. Gamma rays, for example, are electromagnetic, and cause cell mutations that can lead to cancer.
Above is the electromagnetic spectrum. When we’re talking about harm-causing EMF, we look at what’s known as “ionizing” radiation — essentially, the type of radiation that can cause cell mutations and other nasty things. Technically infrafred can cause burns, but we generally don’t care too much about fire when we talk about EMF.
If you look at the picture above, everything to the left of the word ‘ultraviolet’ is ionizing radiation. That means that enough of this can cause cancer. Everything to the right of ultraviolet is non-ionizing, and cannot. Phones operate using a variety of frequencies in the non-ionizing side, like radio. X-rays use ionizing radiation, but at such a low dose that it almost never causes cancer. The sun puts a whole bunch of ultraviolet into our atmosphere, and enough of that definitely causes cancer.