"Subject to them" is the oft-repeated phrase in this Mass. When the Son of God came, He did not exempt Himself from obedience to Divine and human law. He did not live an isolated life away from men when He became the Son of Man.
The Unity of the Holy Trinity was reflected in the unity of the three Persons of the Holy Family, Jesus, Mary, Joseph; each working for the other. This obedience to law (Communion) is singled out as the basis of happiness here and hereafter (Prayer).
The Epistle explains in detail this Christian law for the Christian family, especially charity and unity. Obedience to "My Father's business" (Gospel) must come first, before all other business, if earth and Heaven are to find our family unbroken.