I'm not sure if it's a really popular type. There are so many different kinds of Ben 10 fanfictions out there. Some people might be really into the idea of a baby Ben 10 and write great stories about it, but it might not be as mainstream as other types of Ben 10 fanfictions that focus more on his adventures as a normal - aged hero.
In a fanfiction scenario, Ben and Riley could be at a masquerade ball in their town. They don't recognize each other at first because of the masks. They start dancing and chatting, and there's an instant connection. When they finally take off their masks, they are both surprised and happy, and this event could be the start of a romantic relationship between them.
Well, it could mean that there is a fan - created story about Ben 10 where he is depicted as a baby. Maybe it's a really cute and different take on the Ben 10 character, showing him in his infancy with all the things that come with being a baby, like being small and vulnerable and having to rely on others.
In this fanfiction, Riley discovers she's pregnant. Ben might be hesitant at first because he has some insecurities about being a father. However, as time passes, he sees Riley's strength and determination. They go through the usual pregnancy journey together, like going to doctor's appointments, shopping for baby stuff. Their relationship also deepens during this time as they face the challenges and joys of impending parenthood.
One common theme is family. It shows how Rey and Ben create a family unit with their baby. Another theme is love. Their love for each other deepens as they care for their baby.
In these fanfictions, the theme of legacy often comes up. The baby is seen as carrying on the legacies of Ben and Mal and their respective kingdoms. Another common theme is the adjustment. Ben and Mal have to adjust to their new roles as parents, and the baby has to adjust to the world around it. And of course, there's the theme of hope, that the baby will bring a positive change to Auradon.
In a fanfiction about them as babies, there could be a magical event that only they seem to be aware of. Maybe a tiny fairy visits them in their cribs and gives them a special gift that will influence their lives later. Mal might get a small vial of magic that she'll use in the future, and Ben could get a tiny crown charm. This would be a way to tie their baby story to their grown - up roles in the Descendants world.
Some fanfictions focus on the baby Ben 10 characters being taken care of by other characters. It could be an older version of one of the main characters or a side character who has to babysit them. This leads to all sorts of comical situations as they try to keep the baby versions of the heroes out of trouble. There are also themes about the baby characters' first encounters with new aliens in a more cutesy and less dangerous context compared to the regular Ben 10 stories.
One common theme is his origin and family. Writers often explore how his family background shapes him as a baby. Another theme is his connection to the Force at such a young age.
Family is also a big theme. Maybe they are part of a big family in the fanfiction and the story shows how they are cared for as babies, and how their family interactions shape them. It could be about their parents, siblings, or extended family members and how they all contribute to the baby - Adrian and Ben's growth and experiences.