Present Panic

Bart sat in the living room silently, with four sets of eyes staring at him.  Maybe five—he didn't know what Alfred was doing back in the kitchen.

The group had gotten together so Bart could talk to them about something, but they didn't know what.  Actually, Lois and Diana already knew what was going on; Clark and Bruce were the only ones still in the dark.

"Okay, so, um... I'm from the future."
At this point, the group was completely immune to surprises.  They didn't even flinch at Bart's declaration.  Bart didn't expect that.  He looked around the room at everyone, just to double-check and make sure they'd heard him.  After several uncomfortable seconds of silence, he continued.
"I was born way in the future, but... well, my speed powers have somethin' to do with time, so... well, I don't really understand it all, but they had to send me back in time to make sure my speed didn't get too powerful and end up... uh... killin' me."
"Who's 'they'?" Lois asked.
"I don't remember," Bart replied. "I was eight when they sent me. I remember this robot guy, and a blonde girl, and... that's all.  My Grandma Iris came back from the future with me; I live with her over on the West side of the city."
Clark seemed a little hurt that Bart hadn't been truthful yesterday. "Why didn't you tell us before?"
Bart let his head hang a little.  "I'm... I'm sorry... I just... my grandma told me not to get do anything that'd mess up the future for me, otherwise I might not exist anymore."
On this point, Diana was still a little unclear. "What do you mean?" she asked.
"Well, uh... I saw my grandpa at school today," he answered.
At this, Bruce raised his eyebrow with interest.  Diana helped to explain.  "We met a boy named Barry earlier today.  Bart said that he recognized him as his grandfather, Barry Allen."
Clark put his hand to his chin.  "Now that I think about it, you guys do look a bit alike..."
"Yeah," Bart said.  "And, see, if I do something that messes up the timeline, he might not meet my grandma, and I might not end up being born."
"Hang on," Lois interjected. "Are you getting all your facts about time travel from Back to the Future?"
"That's... that's not the point!  I mean... it's true! If I screw things up here, I might get erased from history!"
Bruce shook his head. "No.  If that were possible, you'd have done it already.  The mere fact that you can still exist in this time means that anything and everything you'll ever do in the present won't stop your future from happening."
The rest of the group spent the next several seconds trying to catch up with Bruce's thoughts.  Eventually, they all reached an "ahhh, I get it" moment.
"So..." Bart said, "I don't have to avoid my grandpa?"
Bruce shrugged. "You can go have lunch with him if you want.  Though you probably shouldn't call him 'grandpa'."