Serah is 15, she's not a child, she is a teenager. She's also living in a world now where age does not matter anymore, no one seemed to give Chuck a hard time when he gave Lee the hard truth and Lee taught Clem everything he knows, so why does Serah get an excuse? She's older and having a disability is a poor excuse.
Because she may literally not be mentally capable of processing what's happening in the way that Clementine is. And if she is, Carlos may have made an honest and understandable mistake in thinking she wasn't.
I don't know a whole lot about this field, but I don't think these sorts of problems can be addressed through a simple attitude adjustment on the part of parent or child. Serah was probably in some sort of Special Education program where Carlos would get regular updates from teachers and specialists on her progress and what she can and can't handle - obviously, that's not happening any more what with the zombies and all.
In the end, Carlos probably made a bad call in trying to keep her shielded from the worst of the situation rather than taking the risk of trying to explain it to her, but that's us speaking with 20/20 hindsight of how it all turned out. He would have been making a difficult judgment call back when this all started.