In the Star Wars saga, particularly in "Revenge of the Sith," Obi-Wan Kenobi's understanding of Anakin Skywalker is deeply complex. He knows Anakin as a friend, a brother, and a Jedi who ultimately fell to the dark side and became Darth Vader. Obi-Wan's emotional conflict stems from the knowledge that Anakin, while capable of great good, is also capable of great evil, especially under the influence of Emperor Palpatine.
Obi-Wan likely understands that Anakin, in his core, would not intentionally harm his children, Luke and Leia. However, as Darth Vader, he is susceptible to the manipulations of Palpatine, who can exploit Anakin's fears, anger, and ambition. This duality creates a tragic scenario where Vader, under Palpatine's influence, could potentially hurt his children—not out of malice, but because he is trapped by his own choices and the dark path he has taken.
Obi-Wan's feelings are further complicated by his sense of duty and loyalty to the Jedi Order, as well as his personal connection to both Anakin and Padmé. He may hope that somewhere within Anakin, there remains a spark of goodness that could eventually lead to redemption, but he also recognizes the danger that Vader poses, particularly in relation to Luke and Leia.
In essence, Obi-Wan walks a fine line between hope and despair, understanding that Anakin's actions as Darth Vader could lead to devastating consequences, even if they are not driven by a conscious desire to hurt his children.