In "Star Wars: Episode III – Revenge of the Sith," after their duel on Mustafar, Obi-Wan Kenobi tells Anakin Skywalker, now Darth Vader, that he has failed. During their confrontation, Obi-Wan does not specifically mention Padmé or their children.
However, in the broader Star Wars saga, Obi-Wan later reveals to Luke Skywalker (in "Star Wars: Episode IV – A New Hope") that Darth Vader was once a Jedi Knight named Anakin Skywalker and that he betrayed and murdered Luke's father. Additionally, it’s revealed to Luke by Yoda that he has a sister, though the specifics of Padmé’s fate and the existence of Luke and his sister Leia are not known to Vader during their duel.
If you had a specific moment or detail in mind from expanded universe media, please clarify, and I can provide more focused information!