Krishna is impartial. He won't save someone just because they were Draupadi's children
I am equally disposed to all living beings; I am neither inimical nor partial to anyone. But the devotees who worship Me with love reside in Me and I reside in them.
- Gita 9.29.
The lifespan of the Upapandavas was up as stated by Lord Shiva
I have protected the Panchalas and have shown him honour. However, they have been overtaken by destiny and can no longer remain alive now."
- 7, Sauptika parva, Mahabharata.
Krishna himself was the all powerful kala that destroyed both the armies in Kurukshetra
All those who have fallen on the field of battle have really been slain by Time. Verily, all of us have been slain by Time. Time is, indeed, all-powerful. You are fully conversant with the puissance of Time. Afflicted by Time, it does not behove you to grieve. Know that Krishna Himself, otherwise called Hari, is that Time with blood-red eyes and with club in hand. For these reasons, O son of Kunti, it does not behove you to grieve for your (slain) kinsfolk.
- Section CXLVIII, Anusasanika parva, Mahabharata.
Markandeya Purana narrates how Draupadi's sons were cursed Visvedevas and were thus destined to die young without marriage and sons
Having heard this their remark, the sage of the Kaushika race, exceedingly enraged, cursed them—“Ye shall all assume human form!” And propitiated by them, the great Muni added, “Although in human form, ye shall have no offspring. There shall be neither marriage of wives for you, nor hostility: freed from love and anger ye shall become gods again.” Thereupon those gods descended to the mansion of the Kurus with their own portions; they were born of the womb of Draupadi as the five grandchildren of Pandu. Hence the five heroic Pandaveyas did not take to themselves wives, through the curse of that great Muni.
- Canto VII, Markandeya Purana.
The destruction of Panchalas and Upapandavas were part of the Bhubharaharana or reducing the burden of earth. First Krishna destroyed the Kaurava army, then he destroyed the Pandava army and finally he destroyed the Yadavas.