Question: Who gave birth to God's spirit children, and if it wasn’t God, the Father, where did the women come from?
Answer: "Gave birth to" is something we don't say with regard to spirit children, since it is not taught that way in any of our scriptures. In my decades as a Mormon, I have never once heard it phrased that way; rather, we speak of spirit children being "created". We have no idea whatsoever how we were created as spirit children, again since that is not explained to us in any detail, at all.
"Created" does not necessarily mean anything remotely related to "giving birth" - and there is no official doctrine of sexual relationships (in the way that they occur here in mortality) in heaven - either prior to the creation of this world or after the resurrection. Unfortunately, that is a common misapplication - among some members (including prior leaders) and nearly all critics, but it is not doctrine. There is no doctrine whatsoever on the process of spiritual creation. This is one case where the proper answer, in my mind, is, "We don't know."
Having said that, I will speculate here. (*grin*)
Genesis 1:27 says, "So God created man in his own image, in the image of God created he him; male and female created he them." We aren't told what that means or how God created male and female in "His" image. Given the totality of the imagery and the familial terminology that is used throughout the Bible to describe our relationships (before our birth, during mortality and after death), we speak of a Heavenly Mother - but there is no obvious, canonized scriptural, unmistakable basis for that belief. (For example, the hymn, "Oh, My Father", includes the following statement: "Truth is reason; truth eternal tells me I've a mother there.")
Personally, I believe that women are created in the image of God, the Mother (my own phrasing and not a title I've heard used in the LDS Church - ever), mostly because of what the Bible says about marriage and our intended end as "one" - as well as statements by our prophets and apostles. Frankly, I believe it is the only way to stay within the Biblical concept of heavenly family and validate women as women - to believe that sex / gender identity has eternal meaning and that women are not simply bastardized, sub-standard men (as has been implied or stated blatantly in many statements by Christian leaders throughout history and as is the natural interpretation of many when the only reference to God is in masculine form). However, I personally do not believe that Heavenly Mother "gave birth" to spirit children, and I personally do not believe that the creation of spirit children involves anything like pregnancy and gestation - in ANY way, shape or form as we know it here on earth.
So, in summary, my own response is that I believe nobody "gave birth to God's spirit children" - that our Heavenly Parents created us in some way, but that it was not in the way children are created here on earth.