I don't know why it is that our lives are filled with children these days - our own grandchild and the grandchildren of many and the children of many - all friends. When I was a child, friends was not automatic. Right now these all seem to be babes in arms and toddlers, but they too must grow into acceptance of the other and all the competitive aspects that arise with respect to character and resources.
It is true too in Genesis that the book is filled with births and with children, not just the ones who are central to the story and who provide the genealogy from Adam to Jacob. Since we began reading on the lectionary 8 or so weeks ago, we have noted Isaac and Ishmael, both blessed, and Isaac living at the well where Hagar was blessed. I asked a scholar at the SBL's Bible Odyssey if there are legends concerning Isaac and Ishmael and why it might be that Beer Lahai Roi (the well of the living one who sees me) would be mentioned these three times in the story - no answer yet.
Targum Jonathan reads: And Izhak was coming from the school of the Rabba Shem, by the way of the fountain where had been revealed to him the Living and Eternal One, who seeth, and is not seen. This transfers to Isaac the revelation to Hagar - strange...
It is notable that the two sons, Isaac and Ishmael are the ones who bury Abraham. So after the separation which Sarah forced Abraham to inflict on Hagar (Genesis 21:10), both children are still in contact. Not enemies yet.
The chosen line is Abraham, Isaac, Jacob. The other children are all over the map and there are a host of them (not included in the lectionary, Genesis 25:1-4). I note that the Targum Jonathan seems to identify Keturah as Hagar.
Abraham had many concubines so many tribes must then be descended from him. Isaac had two sons, Jacob and Esau. Jacob becomes Israel and Esau has children as well - but this is still to come in the story. Isaac will take both a Hittite wife and an Ishmaelite wife. Tribes galore and as today, many unknown futures.