Should adult children who do not wake up for Fajr prayer be punished?

You must advise and remind them, and take steps to help them awaken, such as committing to early sleep and using an alarm clock. If they persist in missing Fajr prayer, then use a combination of encouragement at times and deterrence at other times, even if it leads to depriving them of some of their desires, or punishing them with reprimand and boycotting.