Orthodox Apologetics

Because God is a Trinity, not a Monad.

1. Man and woman are one:

Genesis 2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.

2. Father and Son are one:

John 10:30 I and my Father are one.

3. Mankind is in God's image, the family being the icon of the Trinity:

Genesis 1:27 So God created man in his own image, in the image of God created he him: male and female created he them.

4. Hence why, in conclusion (since children are one flesh with their parents, and wives one flesh with their husbands) the members of a family or household are received communally, and not individually, into the Church:

(See: Acts 16:15; Acts 16:31-34; Acts 18:8; 1 Corinthians 1:16; Hebrews 11:7; 1 Peter 3:20).

When parents entered into covenant with God, they brought their children with them. This was a law in the Hebrew Church. When a proselyte was received into membership, he could not enter without bringing his children with him. -- Easton's Bible Dictionary.