One great fanfiction in this concept might be 'The Young Dark Lord' where a de - aged Voldemort has to deal with a world that is very different from what he remembers. It shows his struggle to regain his power while being in a child's body.
There are not many well - known fanfictions with this exact premise. However, you could try writing your own. Start by thinking about how Voldemort was de - aged. Was it a spell gone wrong? Or a magical experiment? Then consider Harry's reaction. Would he be immediately distrustful or would there be some sort of instinctual connection? You could also think about the implications for the wizarding world. How would Dumbledore react? And what about the other Death Eaters? If you don't want to write it, keep searching on fanfiction archives. Check for new and updated stories regularly as new fanfictions are being added all the time.
The theme of identity is also prevalent. Voldemort has to deal with not being recognized as the great Dark Lord he once was. He has to figure out how to use his new - found 'innocence' as a child to his advantage. And for the other characters, they have to figure out who this strange child really is and what threat he might pose.
One great fanfiction in this genre is 'The Younger Malfoy'. It shows an interesting dynamic between a young Draco and Harry. They end up having to work together in a situation where Draco's de - aging has caused a stir in the wizarding world.
I remember a fanfiction where a de - aged Tom meets Harry much earlier in his life. This creates a whole new dynamic between them. Instead of being the archenemies they are in the original series, they start off as sort of friends - or at least, Harry is determined to befriend Tom and keep him from turning into the evil Voldemort. It shows how small actions and kindness can potentially change the course of someone's life.
It's popular because it offers a new perspective. We get to see the familiar Harry Potter world through the eyes of a de - aged Harry, which is different and exciting.
A really good one is 'Voldemort's Redemption'. This fanfiction flips the script a bit. It explores the possibility of Voldemort being redeemed. It's a fascinating read as it shows how Harry might react to a Voldemort who is trying to change. It also has some great descriptions of the magic and the Hogwarts environment, making it feel like a natural extension of the Potterverse.
One possible fanfiction idea could be that Albus Severus Potter is accidentally de - aged by a misfired spell in Hogwarts. He then has to navigate the school as a much younger version of himself, causing all sorts of chaos and confusion. His friends and teachers are initially shocked but then have to figure out how to reverse the spell.
One common theme is redemption. With Draco being de - aged, it gives him a chance to start anew and make different choices, often leading to a path of redemption. For example, in some fanfictions, he might befriend characters he used to bully.
One great 'de aged Harry Potter fanfiction' is 'The Boy Who Lived Again'. It has an interesting plot where Harry is de - aged and has to deal with the wizarding world in a new way. Another is 'De - Aged Harry's Adventure' which shows his interactions with his friends when he's younger again in a really fun way.
One great 'Harry Potter x Voldemort fanfiction' is 'The Redemption of Tom Riddle'. It explores an alternate universe where Harry tries to understand Voldemort's past and finds a way to redeem him. Another interesting one is 'A Different Bond'. It focuses on a magical bond that forms between Harry and Voldemort, changing the course of their enmity. And 'The Soul's Connection' delves into how their souls, both having a piece of the other, interact in unexpected ways.