The plot could involve some sort of magic gone wrong that turned them into children. In this state, they could be more vulnerable to the threats that usually they can easily handle. Maybe they are hunted by an old enemy who sees this as an opportunity to finally get rid of them. They then embark on a journey to find the source of the magic that changed them and reverse it. Along the way, they learn new things about themselves and their relationship as they are forced to see each other in a different light.
One idea could be that Artemis feels extremely guilty and tries to find a way to reverse the crippling. She might search for rare magical herbs or consult with other powerful deities. Another could be Percy's friends rallying around him, with Annabeth leading the charge to find a solution, while Percy himself struggles with his new limitations and feelings of helplessness.
One possible plot could be that Artemis is captured by some powerful ancient enemy, like a primordial being. Percy, with his skills and courage, embarks on a quest to find her. He might face various challenges and monsters along the way. Maybe he has to solve some riddles related to the moon, as it's Artemis's domain. Finally, he locates Artemis and uses his water powers in a creative way to defeat the captor and free her.
One idea could be that Artemis feels extremely guilty and tries to find a way to reverse the blinding. Maybe she seeks the help of other gods or magical creatures. She could go on a quest to find a rare herb or artifact that has the power to restore Percy's sight.
One possible plot could be Percy getting caught in one of Artemis' hunts by accident. They initially clash due to their different natures but gradually develop feelings for each other.
One possibility is that Artemis has been misinformed. Some enemy of Percy has tricked her into thinking that Percy is a danger to the demigods. So she kidnaps him to stop him from causing harm. But as she keeps Percy captive, she slowly realizes that she has been deceived when Percy starts to explain his side of the story and the real threats he has been fighting against.
A plot could be that there's some sort of dark magic that is affecting the balance between the gods and the demigods. Artemis and Percy are brought together to combat it. Along the way, Percy shows a side of himself that Artemis is drawn to. His courage and selflessness intrigue her. And in a moment of vulnerability, Artemis kisses Percy. This kiss then becomes a catalyst for a deeper exploration of their relationship and the greater threat at hand.
Well, she could betray Percy by revealing his secrets to his enemies. Maybe she's forced to do so under some kind of magical compulsion or threat.
One possibility is that Percy grows up with a strong sense of independence and a connection to nature. Since Artemis is the goddess of the hunt, Percy might be trained in hunting and survival skills from a young age. He could also have a special bond with the animals in the forest, being able to communicate with them to some extent. This upbringing would likely make him a formidable fighter, not just with a sword but also with bows and arrows and stealthy combat techniques.
In this fanfiction, it could be that Artemis is in a situation where she is about to be killed or lose her powers forever. Percy, being the selfless hero he is, takes on a deadly task to prevent that from happening. Maybe it involves facing an ancient and malevolent god who has a vendetta against Artemis, and Percy uses his unique abilities and the element of surprise to try and stop this god, but at the cost of his own life.
One possible plot could be Percy struggling to fit into Artemis's group of hunters. Since he's male and the hunters are usually female, there would be a lot of conflict and misunderstanding at first. He might have to prove himself worthy of being part of the group, not just because he's Artemis's adopted son. He could go on a quest to retrieve a sacred item for Artemis to show his loyalty.