How are we children of God when God's only son is Jesus?
The term son is used equivocally in Catholic doctrine, i.e., in more than way. Jesus is the one and only divine Son of God, eternally begotten of the Father, whom the Father sent to redeem and save us mere created human beings (John 1-3, 14; 3:16-17).
So Christians are God’s adopted sons and daughters because of Christ’s redemptive actions (John 1:10-12; Rom. 8:14-17; Eph. 1:5), which we appropriate beginning with baptism (Titus 3:5; see John 3:16-17; CCC 1265).