Oregon has a mandatory bicycle helmet law for riders under 16 years of age. What that means is that if a child is 15 or younger they are required to wear a helmet if they are riding their bicycle on a highway (Oregon’s term for any public street, not just a major thoroughfare) or on “premises open to the public” which would include publicly owned parking lots, where motor vehicles are allowed, but would not include parks and multi-use paths because they are not open to cars. The law applies to sidewalks, though, because sidewalks are a "portion of the highway" under Oregon law.
If a child is caught riding without a helmet the fine is $25. If the child is 11 years or younger the fine is levied against their parent or another adult legally responsible for the safety and welfare of the child. The adult does not have to be present with the child to be cited.