They might be shocked at first. In the fanfiction, there could be a lot of dark and unexpected plot twists. For example, if their relationships are depicted in a different way or if there are new threats they have to face. They'd probably start by discussing among themselves what they just read, trying to figure out if it has any real meaning or if it's just a wild interpretation of their world.
Raph would be the first to show his concern through anger. He'd be mad at himself for not protecting Leo better and also at the situation that led to Leo's injury. Mikey would be visibly distraught, constantly by Leo's side trying to cheer him up. Donnie would be all business, using his scientific knowledge to figure out the best way to treat Leo's wounds and help him recover.
The other Turtles would probably have a mix of reactions. Leo might be initially concerned about how this would affect their team dynamic and the safety of April and the baby. He would likely try to come up with a plan to protect them. Mikey would be excited and start thinking about all the fun things he could do with the baby, like teaching it to skateboard or play video games. Raph might be a bit more gruff at first, but deep down he would be protective. Splinter would be wise and offer his guidance, seeing it as a new chapter in their family's story.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.