Q. Why doesn’t the book of Luke mention anything about Mary and Joseph taking Jesus to Egypt?
None of the gospel writers are attempting to present a comprehensive account of Jesus’ life, like a modern biography. Rather, they are all selecting and arranging episodes from his life in order to accomplish particular purposes of their own.
Matthew, for example, wants to give particular emphasis to the way that the good news about Jesus is for people of every nation. And so, for example, he includes the account of the Magi coming from “the East” to worship Jesus, while the other writers do not. It makes sense that he would also include a description of how Jesus actually lived in another nation for a time.
Luke, for his part, is writing for educated Greeks within the Roman Empire, and so he includes a long account of Jesus’ journey from Galilee to Jerusalem during which one person after another comes up to Jesus to ask a question or pose a problem, and Jesus responds with divine wisdom. Most of the material that is unique to Luke is found within this journey section, and most of the material within the section is unique to Luke. So we can tell that he has included it with a particular purpose in mind, that of introducing Jesus to wisdom-seeking Greeks.
Luke may well have known about the journey to Egypt. Some of his material seems as if it could only have come from Mary, or at least from people who knew her and passed along her recollections. Those would of course have included the time in Egypt. But as I said, Luke is selecting and arranging his material for a particular purpose, and apparently he did not feel that it was necessary for him to tell the story of the journey to Egypt to achieve that purpose.
Good Question 
Re Miriam’s punishment for speaking against Moses: I checked Strong’s Concordance and could not find any reference to the verb for speaking against Moses being in singular feminine. Where can I find this? Thanks so much.
A concordance will simply give you the meanings of words. It will not show how a given word, in this case a verb, is used in a particular passage, that is, whether it is singular or plural, masculine or feminine, or first, second, or third person. Other than reading in the original Hebrew (where the form is watedabber, third person feminine singular [converted imperfect]), you could use an interlinear Bible. Young’s Literal Translation is also sometimes helpful. In this case it reads, “And Miriam speaketh — Aaron also — against Moses,” showing that Miriam was the lead speaker. You can access Young’s Literal Translation on BibleGateway.com. The default translation there is the NIV, but you can choose a different translation in the pull-down menu just to the right of the orange “SEARCH” box. Hope this is helpful.
The answer to this is in Luke 2:41
They went to Jerusalem every year.
So after Jesus was born in Beth,then presented in Jerusalem, they went back to Nazareth, then one or 2 years later they came back, the wise men came but this time Joseph fled to Egypt with the family