He didn't think anything of it when Sarada asked if she could play with his hair. He was not even suspicious when his niece told him that her dad never let her play with her mom's hair, apperantly he had told her not to even ask Sakura. That should have been the first sign, especially since his brother was currently with the hairstyle he had stopped sporting when he was seventeen. However, Itachi loves his niece too much to mistrust.
At first Sarada put bows in his hair, telling him how pretty his hair was, she even mentioned how her papa's hair was not as soft. All of this made Itachi a little smug, and so when Sarada asked him "uncle itachi, you said I could do anything I wanted right?" That was hint number two, still he ignored his apprehension and nodded to her. She continued with his hair with a renewed vigour.
The third and final hint was practically a confession,and it came too late it occurred some 10 minutes after the second, "did you know that grandmother says she wants you with short hair? She told me she forgot what you look like with short hair, she told me that since she liked how I did papa's hair I should do yours too! I told her that papa hadn't been too happy, but grandma said that you would be!" Well of course, thought Itachi,he could never be mad at Sarada, his mother clearly knew that, everyone knew that. That's when he realized, this was a set up!
Everything suddenly became clear, the moment Sarada had walked into his home she asked to play with his hair, she hadn't even said hello. The haircut his brother had, was not out of nostalgia, or a change, it was Sarada. Sasuke's love of Sakura's hair was no secret, him forbidding Sarada from playing with it was his brother's way of protecting it. All the signs were there, his love for his niece, her innocent smile, how could he have known his mother would corrupt her?
Itachi wondered if it was too late, she had been doing his hair for almost an hour. He imagined his mother's satisfied smirk, she had wanted to see him with short hair for years, and she had been shameless enough to use her four year old granddaughter, her only granddaughter, for this. Itachi couldn't even be mad at Sarada, he told her she could do anything she wanted. He should stop her now, but he was scared to look at the damage, besides the damage was done. He knew Sarada probably cut his hair when she asked him if he was sure. He couldn't be mad, he had no right to be. Sarada was innocent, the real evil, was his mother.
It was then that Sarada announced she was finished. "Uncle itachi, look! You look really pretty!" She handed him a mirror then, he could only smile at her, "do you like it? Grandmother said you would! She said that you would love getting a haircut, I told her that papa didn't seem to like his." Noticing his silence, she asked again, but this time she was genuinely worried, "you don't like it?"
It was not her fault, he told himself, once more, but looking at himself in the mirror it was more difficult to feel the same. His hair was uneven, there were parts that were longer than others,some hair that reached his shoulders, part that didn't even reach his ears. There were mismatched bows all over too. Still he forced himself to smile, and told her, "its just so..." He paused looking for the right word to use, not finding any he tried again, "words cannot describe how much I like this, there are no words." Her smile was wide, and he knew he did the right thing, he would be an adult about this. He could wear a hat until he could get it fixed. "Let's not tell anyone about this ok?"
"That's the same thing papa said!" Itachi was not surprised by her announcement, "can I tell grandmother? She asked me to do this, I want her to know I did, I don't want her to get mad at me."
"You should, let her know I will show her how grateful I am when you see her next time." Poor innocent Sarada simply smiled at him, proud of herself.
"Uncle itachi since you like your hair so much we should go get ice cream! That way others can see it too!"