Why did Avraham request a sign that his descendants would inherit Eretz Yisrael, whereas he did not request a sign about having children?
Rashi #1 (on 15:6): Avraham did not request a sign for Hashem's promise. Rather, he asked in what merit his children would keep Eretz Yisrael, and Hashem answered, "in the merit of the Korbanos."
Ramban (on 15:7): Hashem explicitly promised him children, but regarding the land He said only "I took you from Ur Kasdim in order to give the land to you." Avraham did not request a mere sign, rather, a promise to receive the land.
Seforno: He was concerned lest his children sin and not merit the land.
What was Hashem's reply to Avraham's question?
Ramban: Hashem replied that His promise was unconditional, it was a covenant that was destined to be carried out irrespective of his or his descendants' behavior, or even on the Cana'anim doing Teshuvah. 1
Rashi (to 15:6): They will keep Eretz Yisrael in the merit of Korbanos.
Avraham asked, "By what shall I know that I will inherit it?" There are two ways to explain this question; see Rashi to 15:6. If this is a request for a sign, why did Avraham not lose the reward he had just gained for not asking for a sign regarding the promise of children? According to Rashi's alternate explanation, Avraham asked in what merit they could remain in the Land; yet this is a difficult fit with the language of this verse, "inherit"?
Maharal #1 (Gevuros Hashem, Ch. 8, p. 44): According to the Gemara (Megilah 31b), Avraham's real question was that perhaps his descendants would one day sin and be destroyed, as were the generations of the Flood and Dispersion. Hashem answered that Yisrael would never be destroyed, due to the merit of the Korbanos. When individuals who are intrinsically flawed, commit a sin, Korbanos will not help them. But Yisrael is intrinsically pure and wholesome, and any sin is incidental and not of their essence, and they can atone for it through Korbanos (or nowadays, by learning the laws of Korbanos).
Maharal #2 (ibid. p. 46): Eretz Yisrael is holy and it expels the wicked (Vayikra 18: 24-28). Avraham afraid that this would happen to his descendants.
Maharal #3 (ibid. p. 47): One opinion in the Midrash explains that because almost all people have children, after Hashem assured him of it, he had no questions. But the gift of the Land, which was unique, and Avraham was worried and requested a sign.
Maharal #4 (Derech Chayim 1:2, p. 27): Avodah, i.e. Korbanos, is one of the pillars that the world stands on. This is because a servant has no reality other than his service to his master; similarly, every aspect of Creation exists only inasmuch as it is used in the service of Hashem. This was Avraham's question; if the world was created for His service, and any sinner who deviates from that service will fade from existence, how would his descendants endure? Hashem answered that the Korbanos, which are themselves Avodah, would atone for those sins.
Why did Avraham mention "inheriting" Eretz Yisrael?
Lev Eliyahu (Bereishis p. 107): Avraham's Midah was Chesed. He wanted it to be like an inheritance, whether or not [his descendants] would be worthy of it. 1