This story is not mentioned in the Mahabharata, here is all the Adi Parva of the Mahabharata says about the funeral of Pandu and his wife Madri:
At last they came to a romantic wood on the banks of the Ganga. There they laid down the hearse on which the truthful and lion-hearted prince and his spouse lay. Then they brought water in many golden vessels, washed the prince's body besmeared before with several kinds of fragrant paste, and again smeared it over with sandal paste. They then dressed it in a white dress made of indigenous fabrics. And with the new suit on, the king seemed as if he was living and only sleeping on a costly bed. When the other funeral ceremonies also were finished in consonance with the directions of the priests, the Kauravas set fire to the dead bodies of the king and the queen, bringing lotuses, sandal-paste, and other fragrant substances to the pyre.
Then seeing the bodies aflame, Kausalya [Ambalika] burst out, 'O my son, my son!'--and fell down senseless on the ground. And seeing her down the citizens and the inhabitants of the provinces began to wail from grief and affection for their king. And the birds of the air and the beasts of the field were touched by the lamentations of Kunti. And Bhishma, the son of Santanu, and the wise Vidura, and the others also that were there, became disconsolate.
Thus weeping, Bhishma, Vidura, Dhritarashtra, the Pandavas and the Kuru ladies, all performed the watery ceremony of the king. And when all this was over, the people, themselves filled with sorrow, began to console the bereaved sons of Pandu. And the Pandavas with their friends began to sleep on the ground. Seeing this the Brahmanas and the other citizens also renounced their beds. Young and old, all the citizens grieved on account of the sons of king Pandu, and passed twelve days in mourning with the weeping Pandavas.
The part in bold is the only mention of any role of the Pandavas in the funeral.
The story of flesh-eating is apparently an addition to the Mahabharata found in Telugu folklore, as discussed in this journal paper by T.V. Subba Rao:
The folk created a peculiar story which gives the reason for the tremendous intelligence and immortality of Krishna. Pandu Raja, when he was on his death bed, ordered his sons not to burn his body but to eat it away so that they would become immortal, intelligent and be able to know the future. As soon as he died Krishna arrived on the scene and stopped the Pandavas from eating the dead body. He ridiculed the horrible act of eating one’s own father's corpse and sent them away. The Pandavas left the place keeping Sahadeva near the corpse. Then Krishna entered the dead body without the notice of Sahadeva and began eating it slowly. Meanwhile, Sahadeva was tempted and ate the little finger of the corpse. At once he could know the mischief of Sri Krishna and warned him. By that time, Krishna from within could eat all the parts of the body except the toe of the leg. He came out and cursed Sahadeva like this: "If you reveal my act to anybody, your head will break into thousand pieces!"
Thus Krishna became immortal and shrewdly intelligent knowing all the past, future and present. As he had no time to eat the toe, an ordinary hunter could kill him by piercing an arrow into his toe. Among all the Pandavas, Sahadeva was intelligent because he ate the little finger of the corpse.
So in this version, Sahadeva eats the little finger and Krishna eat almost the entire rest of the body.
In any case, this whole story seems implausible to me for multiple reasons. First of all, in the Mahabharata's account Pandu's body seems to be intact at the time of cremation. Second of all, this story seems entirely contrived to explain three facts which need little explanation:
Krishna being shot by the hunter Jara
Krishna's intelligence needs no explanation; he's an incarnation of Vishnu, after all. Sahadeva's intelligence can be explained by the Ashwini Kumaras' association with intelligence. And the causes of Krishna being shot by the hunter Jara have been extensively described in the Mahabharata, as I discuss in this answer.