Because
Disclaimer: Sadly I don't own Teen Titans. Might be looked as Raven/Robin friendship or romance pairing, you decide.
Quote: Raven speaking to herself in the train with her children: "Yes Robin, pick me to pick up three children."Episode 59: Hide and Seek
Raven went downstairs to the living room to find Robin alone, watching TV. She really wanted to know why. Why Robin picked her to pick up Melvin, Teeter, and Timmy, or her children as she often thought of them.
"Sometimes Boy Wonder, I often wondered what is wrong with your judgment?" asked Raven as she stepped into the light, so Robin could see her.
"There's nothing wrong with my judgment" Robin replied, as he watched her face. Raven was trying to put on a neutral face, but he could have sworn he saw the sides of her lips turn to a small smile.
"So, why did you make me pick up my children?" asked Raven.
Robin knew why, and he wanted Raven to see the reason as well.
"Because" he paused for a second and watched as Raven shifted on the couch "Because I knew you could do it."
"Right, Boy Wonder, so why did you never let me talk to you during the mission, how was I suppose to know how to take care of my children, and was it necessary for Beast Boy to use your communicator?" asked Raven.
"I just knew you could do it, I mean, you call the kids your children, and I knew along. As for why Beast Boy had the communicator, I just knew you didn't really want to do the mission" replied a smirking Robin.
Raven threw a pillow at Robin's head with using her powers and laughed a small laugh as she left the room.
Oh yes, the kids reminded her she wasn't a bad person, just dark at times, but really she was good.
Robin stayed in the living room, so he couldn't confront another pillow in his face, but he didn't see Raven leaving the room with the smile on her face.
Robin just knew that because Raven could actually deal with kids. 'She'll make a great mother someday, she just didn't know that before this…'thought Robin.