You can try Archive of Our Own (AO3). It has a vast collection of fanfictions including ones about Isaac and Stiles having a baby. Just use the search bar and type in relevant keywords like 'Isaac Stiles baby' and you'll likely find a lot of stories.
One great 'baby Isaac fanfiction' could be about his daily adventures in a magical world. Maybe he discovers a hidden forest filled with talking animals and has to find his way back home.
One great story could be 'The Unlikely Bond'. It focuses on how Isaac and Stiles navigate the new situation of having a baby. It shows their growth as characters and how they deal with the challenges of parenthood. The story is full of heartwarming moments and also some tense ones as they face society's reactions.
First, think about the character of baby Isaac. What are his traits? Is he curious, shy, or brave? Then, come up with a simple plot, like a trip to the park where something unexpected happens.
First, research the 'Teen Wolf' universe where Isaac Lahey is from. Know the rules of the supernatural world in it. Then, develop a strong personality for his sister. Is she brave like Isaac? Or more shy? Next, create a unique conflict. It could be something like she's being targeted by a new supernatural threat because of her relation to Isaac. And don't forget to add some emotional depth, like showing how much she loves and looks up to her brother.
Some popular baby Isaac fanfics often revolve around his growth and development. For example, there could be a fanfic that shows how he interacts with his environment as a baby. It might describe how he responds to different sounds, colors, and people. Another type could be centered around his family's love for him during his infancy, like how they take care of him and the special moments they share.
The relationship between Pack Mom and Isaac could develop through acts of care. For example, Pack Mom cooks for Isaac, helps him with his homework if he's still in school in the fic. This makes Isaac trust her more. Stiles and Isaac's relationship might start with Stiles being a bit wary of Isaac as a new addition to the pack. But as they go on adventures together, like facing a minor werewolf problem in the neighborhood, they become closer friends. And as for Pack Mom and Stiles, they could have a deeper bond as they both care for Isaac.
One idea could be that Pack Mom takes in Isaac when he's injured. She nurses him back to health and in the process, Isaac starts to see her as a mother figure. Stiles is initially jealous but then realizes how much Isaac needs that maternal love. Another plot could involve a threat to the pack where Pack Mom, Stiles and Isaac have to work together to protect the others. Isaac discovers a new power during this time which helps them in the fight.
Tumblr can be another source. There are many fanfic writers who post their works on Tumblr, and by using the right tags such as 'baby Isaac fanfic', you might be able to discover some interesting reads. However, the content on Tumblr can be a bit more scattered compared to AO3 and Fanfiction.net, so you might need to do some more digging.
One idea could be that Isaac's baby sister discovers she has a special power. Maybe she can communicate with animals and it causes all sorts of chaos and also helps in unexpected ways. For example, she might use this power to find a lost pet in the neighborhood which makes her a local hero among the kids.