Bildad also tried to convince Job that Job was a sinner. So Bildad's expression that his children were sinners and got what they deserved doesn't exactly resonate with me. In chapter one where it discusses the children's feasting, it doesn't show any signs of disapproval of the feasting itself. We see where the brothers invite their sisters to it because otherwise they likely wouldn't go because of a humble spirit. If anything, I would think that chapter one is showing us that Job had a family that enjoyed spending time with one another. Contrast this to Jacob's children and how they plotted against one another.
Another thing to remember is that Job made sacrifice for his children. I personally believe that the events of this book take place prior to the Mosaic law and therefore the head of the family acted as the high priest for the family. Any sins that they may have committed were taken care of by Job's sacrifices. The reasoning for his sacrifices were given as well. Job was aware of their fellowship, but did not take part. I don't know why he chose to not take part, and that could be indicative of a disapproval, but not necessarily. Regardless, he sacrificed just in case they blasphemed "in their heart", not because of their fellowship and feasting. If they died because of sin as Bildad suggests, then that would suggest that Job was not taking his responsibility as high priest of his family very seriously and that he failed to sacrifice for them appropriately. It would potentially mean that his sacrifices for them were tainted by sin which we know is not the case.
The book of Job doesn't really make it clear whether or not the children were in sin. But that's not the purpose of the book of Job. The book of Job, according to James was to show the endurance of Job and the end result that God had in store for Job.
James 5:11 KJV
Behold, we count them happy which endure. Ye have heard of the patience of Job, and have seen the end of the Lord; that the Lord is very pitiful, and of tender mercy.
We see the innocence of Job throughout the book. His children and their virtue or lack thereof is not part of the scope of the book and therefore not really clarified on much. We have to take hints given throughout to determine whether or not his children died as a result of sin. And even then, we may still not know the mind of God on this matter. The book of Job proves that sometimes, people suffer for other reasons than because of sin. Like Abel, people suffer due to the unrighteousness of others. Like Paul, we suffer so that we may be a testament and a witness to others. Like Job, we may be allowed to suffer to show principalities and powers in heavenly places the manifold wisdom of God (Ephesians 3:10). Suffering is also a means that God uses to cause us to grow and mature in our faith, and sometimes, he uses it to get our attention and turn back to Him because we have sinned.
I hope this helps!