Why didn’t the heroes try to use Eri to rewind Nighteye to prevent him from dying?
There are multiple reasons behind this:


Quirk. As @Kerkhof &amp; @ConMan said, she can't control her powers and could do more harm then good. We still don't know the extent of her quirk so she could turn in into a baby in 3 seconds if she loses control maybe.



   



Eri's state. After the fight with Chisaki, she is seen in a deep sleep/"coma" like state and they could do nothing to wake her up so she could'nt help even if she wanted to.



   



Time. We see in chapter 161 that Night-Eye dies moments after they are all rushed to the nearest hospital so even if Eri was conscious and healthy and even if they were able to find a way to use Eri's powers to help Night-Eye, it would've been a race against time and there were no assurances that they could do it.
Trauma: Everyone in knew how much Eri suffered. And how she blamed herself for everything. Seeing this people decided not to tell Eri about Night-Eye's Death. Because she is too small to understand/process all this. She might have even started hating herself or go berserk.(context: Anime)