A boy in a shower with a grown man is a reasonable suspicion.
It shouldn't be suspicion at all, but some observable evidence, and that evidence could be physical or behavioral or some combination, but suspicion is not a word that belongs in the standards for deciding when to call law enforcement.
There are plenty of horror stories around involving child protective services who acted on the suspicion of some busy body who is always suspicious, and who feels like the world is in need of their judgment and wisdom.
Of course, use of the word "suspicion" in statutes comes from the some sort of folks who gave us the "Affordable" Health Care Act.