Were all sons of Pandavas (from Draupadi and other wives) killed?
No, all sons of Pandavas were not killed. As per Adi Parva Chapter 95, Pandavas other than Arjuna had the following sons
Besides these, Yudhishthira, having obtained for his wife Devika, the daughter of Govasana of the Saivya tribe, in a self-choice ceremony, begat upon her a son named Yaudheya. And Bhima also obtaining for a wife Valandhara, the daughter of the king of Kasi, offered his own prowess as dower and begat upon her a son named Sarvaga
And Nakula obtaining for his wife Karenumati, the princess of Chedi, begat upon her a son named Niramitra. And Sahadeva also married Vijaya, the daughter of Dyutimat, the king of Madra, obtaining her in a self-choice ceremony and begat upon her a son named Suhotra.
There is no mention of these sons anywhere other than Adi Parva.
And there is also Babruvahana, son of Arjuna and Chitragandha, who didn't take part in Kurukshetra battle and had later defeated Arjuna.
And the following were killed in/after Kurukshetra
From Yudhisthira - Prativindhya
From Bhima - Sutasoma
From Arjuna - Srutakriti
From Nakula - Satanika;
From Sahadeva, Srutakarman
The above were killed by Ashwathama
Subhadra and Arjuna - Abhimanyu (killed by Durmasana, son of Dushasana)
Ulupi and Arjuna - Iravan (killed by Alambusha)
Hidimva and Bhima - Ghatotkacha (killed by Karna)
So altogether there were 13 sons of Pandavas, and 8 of them perished in Kurukshetra battle
It seems that that is the case.
Here are references from Mahabharatam, Mandalam 10, Vibhaagam 8, narrated by Sanjayan:
... he beheld Uttamauja sleeping on his bed. Attacking him with his foot on the throat and chest, Drona's son slew that great hero also while the latter writhed in agony. Yudhamanyu, coming up and believing his comrade to have been slain by a rakshasa, speedily struck Drona's son in the chest with a mace. Rushing towards him, Ashvatthama seized him and brought him down to the ground and slew him like an animal while the latter uttered loud shrieks.
Having slain Yudhamanyu thus, that hero proceeded against the other car-warriors of the king, who were all asleep. ... Covered all over with blood, he seemed then to be Death himself commissioned by time.
Of terrible form, he careered in the camp like Yama himself, and at last saw the sons of Draupadi and the remnant of the Somakas. Alarmed by the noise, and learning that Dhrishtadyumna had been slain, those mighty car-warriors, the sons of Draupadi, armed with bows, fearlessly poured their shafts on Drona's son. Awakened by their noise, the Prabhadrakas with Shikhandi at their head, began to grind the son of Drona with their arrows. Drona's son, beholding them shower their arrows on him, uttered a loud roar and became desirous of slaying those mighty car-warriors. Recollecting the death of his sire, Ashvatthama became filled with rage. Alighting from the terrace of his car, he rushed furiously (against his enemies). Taking up his bright shield with a 1,000 moons and his massive and celestial sword decked with gold, the mighty Ashvatthama rushed against the sons of Draupadi and began to lay about him with his weapon. Then that tiger among men, in that dreadful battle, struck Prativindhya in the abdomen, at which the latter, O king, deprived of life, fell down on the Earth. The valiant Sutasoma, having pierced the son of Drona with a lance, rushed at him with his uplifted sword. Ashvatthama, however cut off Sutasoma's arm with the sword in grasp, and once more struck him in the flank. At this, Sutasoma fell down, bereft of life. The valiant Shatanika, the son of Nakula, taking up a car-wheel with his two hands, violently struck Ashvatthama at the chest. The regenerate Ashvatthama violently assailed Shatanika after he had hurled that car-wheel. Exceedingly agitated, Nakula's son fell down upon the Earth, upon which Drona's son cut off his head. Then Shrutakarma, taking up a spiked bludgeon, attacked Ashvatthama. Furiously rushing at Drona's son, he assailed him violently on the left part of his forehead. Ashvatthama struck Shrutakarma with his excellent sword on the face. Deprived of senses and his face disfigured, he fell down lifeless on the Earth. At this noise, the heroic Shrutakirti, that great car-warrior, coming up, poured showers of arrows on Ashvatthama. Baffling those arrowy showers with his shield, Ashvatthama cut off from the enemy's trunk the latter's beautiful head adorned with ear-rings. Then the slayer of Bhishma, the mighty Shikhandi, with all the Prabhadrakas, assailed the hero from every side with diverse kinds of weapons. Shikhandi struck Ashvatthama with an arrow in the midst of his two eyebrows. Filled with rage at this, Drona's son, possessed of great might, approached Shikhandi and cut him into twain with his sword. Having slain Shikhandi, Ashvatthama, filled with rage, rushed furiously against the other Prabhadrakas. He proceeded also against the remnant of Virata's force.
At the end of this vibhaagam, Ashwatthama says, (emphasis mine)
In great joy the latter uttered these words: "All the Pancalas have been slain, as also all the sons of Draupadi! All the Somakas also, as well as all that remained of the Matsyas, have been slaughtered by me! Crowned with success, let us without delay go there where the king is! If the king be still alive, we will give him this joyful intelligence!"