It's unreasonable to believe that a small child can run faster than a police officer.
Literal
Young child [subject-が] police-officer than run [nominalizer-の] [subject-が] fast [quotative-と] believe [nominalizer-の] [topic-は] reasoning [to-に] agree-not.
A layered sentence with multiple nominalizations. 理屈に合わない ('not in accord with reason / unreasonable') is a set expression. 走るのが速い ('fast in running') is a nominalized comparison structure. Dense parsing required.