In Canada, the communication I've been seeing is basically that they are only interested in testing you if you might need treatment. Since people under 18 are at such low medical risk from this virus, they're probably very low priority to test. Combine that with the fact that laws probably exist that make it inconvenient to provide this service for them, and it likely makes sense to just blanket exclude them from the program.

As far as I know there is no treatment for COVID-19. Why do they need testing to determine treatment?

There is absolutely treatment for the disease. It's just that the treatment is largely restricted to ameliorating the symptoms while the root cause, the virus proliferation, is left to the immune system. (The possible exception are antivirals currently being tested.) Symptom treatment includes things like concentrated oxygen and mechanical ventilation.

Some cases are getting remdesivir which looks pretty promising.

There are trying to measure how fast it is spreading in different areas so they can take appropriate counter measures to avoid overwhelming the health care system in those areas. i.e. "flattening the curve"

They have to agree to the Google Terms of Service?

I just signed up, and no, I did not have to agree to a Google Terms of Service. Of course, I did when I got a Google account.