The child's character can be developed by giving them unique goals. For example, if they want to become a better ninja than their parents, it would drive their actions and decisions. Also, showing their relationships with other characters, like their grandparents or their parents' friends, can shape their personality. Their reactions to different situations, such as facing a difficult mission or a family argument, are important too.
One possible plot could be that Sasuke is constantly away on missions, leaving Sakura to take care of their second child mostly on her own. This could lead to some internal struggles for Sakura as she tries to balance her medical ninja duties with motherhood. And the second child might start to feel neglected by Sasuke's absence and cause some family drama.
A possible plot is that the child is born with a unique power that neither Sasuke nor Sakura have. This power could attract the wrong kind of attention, and the child has to learn to control it while also hiding it from those who would misuse it. The story could also involve the child making friends with kids from different ninja clans and trying to bridge the gaps between the clans, just like their parents did in a way. Also, the child might have to deal with the pressure of being the heir to two very powerful ninja legacies.
Sakura would likely develop more patience and strength as she manages a growing family. She's already a strong character, but with a second child, she'll have to learn new ways to handle the chaos. Sasuke, on the other hand, might become more in touch with his emotions as he sees his family expand. He could start to understand the importance of being present more often.
One idea could be that their child discovers a new, unique chakra ability. Since both Sasuke and Sakura are powerful ninjas, their genetic combination might lead to something extraordinary in their child. For example, the ability to combine Sakura's medical ninjutsu with Sasuke's Sharingan - like being able to see the internal damage of an enemy through the Sharingan and heal it instantly with a new form of medical jutsu.
The character could be developed through their relationships. For instance, their relationship with their parents. If Sasuke is still somewhat distant due to his past, the child might strive to get closer to him, which would shape their personality. They could also have a strong bond with Sakura, learning medical ninjutsu from her at an early age.
The child might have Sasuke's dark hair, which is a very prominent feature in his family. And perhaps Sakura's green eyes, which would be a beautiful combination. They could also inherit Sasuke's angular facial features along with Sakura's rosy cheeks.
One possible plot is that the second child has a different personality from their first. The child could be more outgoing and curious, constantly getting into minor troubles in the village. Sasuke, being the more reserved parent, might struggle to understand the child while Sakura tries to bridge the gap between them. It could also focus on how the family adjusts to having a new addition and how the second child's birth affects the family's standing in the village.
In fanfiction, Sasuke is often depicted as a strict yet loving father. He might be distant at times because of his past, but he always has his family's best interests at heart. For example, he could be strict about his child's ninja training to ensure they can protect themselves. Sakura, as the mother, is usually the more affectionate one, always there to comfort the child when they're sad or hurt.
It's possible that the second child's abilities are more latent at first. While the first child might have shown early signs of great power, the second child could seem ordinary in the beginning. However, as the story progresses, they could develop an ability that is even more powerful than the first child's. For example, the second child might discover a new form of chakra control that is unique to them.