If parents are muslim but the child, major, decided to leave the religion of his parents, are the parents judged or is it a sin for them ?
Obviously the parents who did not give their children any Islamic education or gave them false information share part of the sin of their children leaving Islam. This is because they did not fulfill their responsibility as parents to educate their children in the religion.
However, those parents that did fulfill their responsibility and taught their children Islam and the Quran, will not be held responsible for their children's sins.
Allah says in the Quran:
[...] Each soul is responsible for its own actions; no soul will bear the burden of another. You will all return to your Lord in the end, and He will tell you the truth about your differences. (Quran 6:164)
Even in the case above where the parents share the sin, they are sinful because they failed to fulfill their own responsibilities meaning it is their own actions they are sinful for.
The Quran, in fact, gives the story of the Prophet Nuh (AS) who was unable to guide his own son:
And it (the Ark) sailed with them through waves like mountains, and Noah called to his son who was apart [from them], "O my son, come aboard with us and be not with the disbelievers."
[But] he said, "I will take refuge on a mountain to protect me from the water." [Noah] said, "There is no protector today from the decree of Allah, except for whom He gives mercy." And the waves came between them, and he was among the drowned.
And it was said, "O earth, swallow your water, and O sky, withhold [your rain]." And the water subsided, and the matter was accomplished, and the ship came to rest on the [mountain of] Judiyy. And it was said, "Away with the wrongdoing people."
And Noah called to his Lord and said, "My Lord, indeed my son is of my family; and indeed, Your promise is true; and You are the most just of judges!"
He said, "O Noah, indeed he is not of your family; indeed, he is [one whose] work was other than righteous, so ask Me not for that about which you have no knowledge. Indeed, I advise you, lest you be among the ignorant."
[Noah] said, "My Lord, I seek refuge in You from asking that of which I have no knowledge. And unless You forgive me and have mercy upon me, I will be among the losers."
This story shows that it doesn't matter how pious the parents are, the children can choose to be misguided. Even a Prophet who was the best of people in preaching the religion could lose his son to misguidance.