Why does God command us to have children? Genesis 1:28
  
  Easy-to-Read Version (ERV)
  
  28 God blessed them and said to them, “Have many children. Fill the
  earth and take control of it. Rule over the fish in the sea and the
  birds in the air. Rule over every living thing that moves on the
  earth.”


There are so many Christian concepts that make me scratch my head as to why God wants us to have children. Things like the ruler of this world is Satan, Christians must suffer in this world, we are anxiously waiting for Jesus' return and the rapture, fighting our carnal nature of a fallen world....etc. Wouldn't it be better to leave unborn children wherever they may originate from?

So what is the reason God wants us to have children?

(Also, if you happen to known - does God NOT want non-Christians to have children?)
The verses you quote give one very obvious reason: to fill the earth, take control of it, and rule it. 

This might be slightly more understandable by analogy to adopted children of a good king. The king desires his children to not just like him in title (accomplished simply by the adoption) but like him in character and action (rightly using authority develops right character and right character motivates a caring that demands that authority be exercised and exercised rightly). 

(Part of this rule would seem to be to command the obedience of "lower creatures" with respect to their being fruitful. [God's command to the sea and sky creatures in Genesis 1:22 to "increase" is implicit to other creatures.] This would seem to imply that human reproduction which prevents such obedience by destroying habitats would be contrary to the command.)

One aspect of being made in the image of God is that human's are meant to be like images of a King scattered about the kingdom to declare the authority of the King. Images that more faithfully reflect the original more clearly make that declaration. (Declarations of autonomy--e.g., my labeling myself [as a statue] "King Paul"--would be perhaps the most outrageous violation of that purpose.)

Parenting is also one of the ways that God helps humans understand his character (being like a husband and like a father to his creation but also to understand the Father's love to his adopted children that he would give his begotten Son for them), develop a like character, and display his character (in miniature and like a statue or picture). Even the act of procreation itself mimics (as a child imitating the parent) God's act of creation. It also seems that parenting wears very abrasively against self-will and self-sufficiency and more strongly forces prioritization of the use of time and other resources.

Biological reproduction is also a reasonably effective form of evangelism, of growing the kingdom.

With respect to Satan as the prince of this world, the Christian is comforted that Jesus has overcome the world (John 16:33b--"In this world you will have trouble. But take heart! I have overcome the world.", NIV). To reject children out of fear of Satan would be to reject this comfort.

With respect to the waiting for the return, the Christian is called to be active and fruitful in this time. Burying one's talent to be guaranteed not to suffer loss seems to be viewed rather harshly in the Parable of the Talents (Matthew 25:14-30).

With respect to fighting our carnal nature, Biblical Christianity is not like that form of Gnosticism that taught that the spirit is good and the matter is evil. In fact, the Bible teaches that the Christ ascended into heaven with a physical body (which ate food presumably with fleshly pleasure). While sexuality is a major area of human corruption, it is part of the created order that God originally declared "good". By properly exercising sexuality, a couple declares God's redeeming power over this prominent area of fallenness. (For Roman Catholics, who reject the separation of the unitive and reproductive aspects of sexuality, the connection of reproduction with proper exercise of sexuality may be more explicit; but I suspect even most Protestants would not entirely separate sexuality from reproduction, at least recognizing that there is a creaturely desire to procreate that is linked with sexuality and that the full function of sexuality involves reproduction.)