I’ll try to put this in words a six-year-old can understand (so they may not be as theologically precise as if I was writing to an adult).
The relationship between God the Father and God the Son is not exactly like the relationship between a human daddy and a human child. With human daddies and children, daddies lived for a long time (usually many years) before their children were born. With God, there is no time. All that happens occurs in an eternal Now. That means that there was never a time that God the Father and God the Son did not exist. It also means that God the Son proceeded from God the Father in a far different way than children are born of their mommies and daddies.
We call the Father and Son “Father and Son” because the relationship between human daddies and children is something like the relationship between the Father and the Son. Mainly it is because the Son is the image of the Father, just like human children can look a lot like their daddies.
When God decided to fix the relationship between himself and humans, he chose to become human in the womb of his human mother, the Blessed Virgin Mary. This action did not require the help of a human father. However, God knew that the Virgin Mary and Jesus would need a human father to protect and provide for them. So, an angel appeared to St. Joseph and let him know that God had chosen him for this task. That is why St. Joseph and the Virgin Mary wed.
Why is it that God sometimes does things in a different way than he expects of humans? Basically, because he is God and we are humans. While he has made us to be like him, we can never be God, and so sometimes what God does is different than what humans do.
