What causes some children to endure challenging experiences, such as early family separation or lack of exposure to religion? Is it possible that these hardships result from their parents' sins and failure to repent to Allah?
Despite the fact that bad deeds of parent can leave bad impacts on their children, but it is not the only real reason always. There are very pious parents like prophet Noah as a Quranic example who had a disobedient son despite all his kindness, wisdom and nice manners. Some times grown up children fall under influence of their evil desires and Satan and bad company and they don't follow their pious parents.
Parents should treat their grown up children in a friendly way and avoid enforcing attitude which might create a bad reaction from the children against their parents.
Parents need to always try to protect their children from bad friends as much as possible.
Parents need to always pray for their children to be saved from Satan and Satanic people.
There are many supplications in Quran and Hadeeth in this matter.
Wassalam.