Sam would probably be very protective. He has always been there for Dean, and now with a new addition to the family in the form of a baby, he would see it as his responsibility to keep the baby safe. He might also be a bit of a mentor, teaching the baby about the world of hunting and how to handle different supernatural creatures.
At first, Sam might be a bit worried. He knows the dangerous world they live in and how having a baby in such a situation could be risky. But as time goes on, he would warm up to the baby completely. He would play with the baby, tell it stories of his and Dean's adventures, and be an important part of the baby's life. He would also be on the lookout for any threats that might come towards the baby due to its unique parentage.
There's also 'Dean, Castiel and Baby: A New Bond'. This fanfiction focuses on how Castiel, in his new 'baby' form, starts to understand human emotions better through his interactions with Dean. It has a unique take on their relationship and is quite engaging.
One key element is the relationship dynamic. Dean is often portrayed as the protector of baby Castiel. For example, in many stories, Dean will go to great lengths to keep Castiel safe from harm.
Castiel first wakes Dean gently from the nightmare. Then he sits with Dean and tells him stories about the great heroes in Heaven. He tells Dean that he is also a hero, and that no nightmare can ever truly harm him because Castiel will always be there to protect him. He strokes Dean's hair softly as he talks, which calms Dean down.
One great 'dean and baby sam fanfiction' is 'The Early Days'. It beautifully portrays Dean's protective nature towards baby Sam. Another is 'Sam's First Steps in Dean's Care' which focuses on the little moments of Sam growing up under Dean's watchful eye. And there's also 'Dean and Sam's Bond from the Start' which delves deep into their emotional connection right from when Sam was a baby.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'dean and baby sam fanfiction' stories. Just use the search bar and type in relevant keywords like 'dean and baby sam' and you'll get a bunch of results.
One common theme is protection. Dean is often portrayed as extremely protective of Sam. For example, in many stories, he will go to great lengths to keep Sam safe from any harm.
One common theme is family. The baby bump represents a new addition to their family, and it often shows how Dean and Sam redefine their idea of family. Another theme is protection. Dean is usually very protective of Sam when he has the baby bump, which shows their strong brotherly bond.
One common theme is redemption. Demon Dean often has a chance to redeem himself through his relationship with baby Sam. Another theme is protection. Dean, even as a demon, feels the need to protect Sam. For example, in many fanfictions, he'll fight off other demons who try to harm Sam.
The baby could have the power of teleportation. Since both Dean and Castiel are involved in a world full of supernatural elements, teleportation would be a useful power. It could help the baby escape dangerous situations or quickly reach its parents when they are in trouble.
The theme of family or a found family could be prominent. Despite not being related by blood, Castiel acting as a daddy to Dean creates a family - like bond. Also, the theme of healing is possible. If the nightmares are a result of emotional wounds, then the story could be about how Dean heals with Castiel's help.