Why is Christ called the only begotten Son of God, since we are also the children of God?
Because Christ alone is the eternal and natural Son of God, but we are children adopted of God by grace for His sake.
Scripture references
Why do you call Him our Lord?
Because He has redeemed us, both soul and body, from all our sins, not with silver or gold but with His precious blood, and has delivered us from all the power of the devil, and thus has made us His own property.