In II Samuel 7:12 states: "When your days are fulfilled and you lie down with your ancestors, I will raise up your offspring after you, who shall come forth from your body, and I will establish his kingdom" (NRSV).
According to P. Kyle McCarter, this originally referred to David's seed in a collective sense, as in his entire royal dynasty, but the Deuteronomist inserted v 13 to apply it specifically to Solomon.
Here, I want to look at the phrase that the NRSV translates "from your body" (mi-memecha) to see if that is used in the context of a collective seed, the immediate offspring, or both.
Genesis 15:3-5: And Abram said, "You have given me no offspring, and so a slave born in my house is to be my heir." But the word of the LORD came to him, "This man shall not be your heir; no one but your very own issue shall be your heir." He brought him outside and said, "Look toward heaven and count the stars, if you are able to count them." Then he said to him, "So shall your descendants be."
This seems to relate to Abraham's immediate offspring, Isaac, who is the prerequisite for other descendants. The idea is that Abraham will have a son, who will be his heir.
Genesis 25:23: And the LORD said to her, "Two nations are in your womb, and two peoples born of you shall be divided; the one shall be stronger than the other, the elder shall serve the younger."
Esau and Jacob were in her womb, yet, in a sense, they are collective, since they are the ancestors of nations.
Numbers 5:22: now may this water that brings the curse enter your bowels and make your womb discharge, your uterus drop!" And the woman shall say, "Amen. Amen."
The woman won't be able to have immediate offspring because of the water she drinks.
Ruth 1:11: But Naomi said, "Turn back, my daughters, why will you go with me? Do I still have sons in my womb that they may become your husbands?
This is immediate offspring.
II Samuel 16:11: David said to Abishai and to all his servants, "My own son seeks my life; how much more now may this Benjaminite! Let him alone, and let him curse; for the LORD has bidden him.
Immediate offspring, namely, Absalom.
II Chronicles 32:21: And the LORD sent an angel who cut off all the mighty warriors and commanders and officers in the camp of the king of Assyria. So he returned in disgrace to his own land. When he came into the house of his god, some of his own sons struck him down there with the sword.
Immediate offspring.
Psalm 71:6: Upon you I have leaned from my birth; it was you who took me from my mother's womb. My praise is continually of you.
Immediate offspring.
Isaiah 48:19: your offspring would have been like the sand, and your descendants like its grains; their name would never be cut off or destroyed from before me.
Different versions translate this in different ways. The NRSV has descendants, whereas the NAB has "those born of your stock." Is this immediate offspring or collective seed? A little of both, perhaps, since the topic is the Israelites having lots of children for the nation.
Isaiah 49:1: Listen to me, O coastlands, pay attention, you peoples from far away! The LORD called me before I was born, while I was in my mother's womb he named me.
Immediate offspring.
Conclusion: It mostly means immediate offspring, but it can be collective and refer to descendants, every now and then. So why's McCarter see it as collective? Maybe on the basis of context, since II Samuel 7 promises David an everlasting dynasty. And, for there to be an everlasting dynasty, God must only chastise the descendants of David, not remove them from the throne. But here's a thought: Maybe the Deuteronomist thought that whole chastisement business only applied to Solomon, but not Solomon's descendants who came after him. In that case, he thought Solomon was the one with the unconditional covenant, whereas Solomon's descendants had a conditional covenant: God would preserve their line if they obeyed.
But that may not work, for I Kings 9 says God will preserve his covenant only if Solomon and his descendants obey, so Solomon doesn't get a free pass, while his descendants are saddled with conditions. Plus, David in I Kings 2 exhorts Solomon to keep the commandments so God can maintain his covenant.
Now I want to take a look at the parallel term used in I Kings 8:19: nevertheless you shall not build the house, but your son who shall be born to you shall build the house for my name.' This literally says "from your loins" (me-chalatsecha).
Genesis 35:11: God said to him, "I am God Almighty: be fruitful and multiply; a nation and a company of nations shall come from you, and kings shall spring from you.
Here, it's collective. Descendants are said to come from Abraham's body.
Conclusion: I guess I Kings 8:19 is simply using an expression that's interchangeable with that used in II Samuel 7:12. While immediate offspring can be said to come from a person's body (be it man or woman), so can descendants. So a phrase about coming from a person's body doesn't have to mean an immediate son or daughter, but it can refer to descendants as well. II Samuel 7:12 is therefore ambiguous, but the Deuteronomist took it in a specific way, according to his agenda.