Aqua, if she were involved, would be determined to save Kairi. She has a strong sense of justice and protecting the innocent. She would likely use her magic to try and track Xehanort and Kairi. Donald and Goofy would be loyal to Sora and follow him into the fray, but also try to keep Sora from making hasty decisions. They would offer their support in combat and also try to think of practical ways to free Kairi, like finding Xehanort's weaknesses.
One possibility could be that young Xehanort grabs Kairi as part of a plan to manipulate Sora. He might think that by having Kairi in his control, he can draw Sora out and into a situation where he can be easily influenced or defeated. This could lead to a series of events where the heroes have to figure out how to rescue Kairi from Xehanort's clutches while also dealing with his larger scheme.
He might capture her for power. Kairi is an important figure, and her capture could give him an edge.
Well, it could be that Young Xehanort captures Kairi to gain some kind of power. Since Kairi has her own unique abilities and significance in the Kingdom Hearts universe. He might think that by having her in his control, he can manipulate events to his advantage. For example, he could try to use her to access certain keyblade powers or to open paths to different worlds that are otherwise inaccessible.
Kairi could be looking for an opportunity to use her physical combat skills. If she can get close enough or find a moment of weakness in Young Xehanort's grip, she might try to land a blow or use a self - defense move to break free. She may have learned some basic combat moves from training with Sora and Riku, and she could be relying on those in this situation.
One possible reason could be that Young Xehanort is very powerful. His strength might be overwhelming for Kairi, both physically and in terms of his dark magic if he uses it against her. Another reason could be that Kairi is in an unfamiliar situation. Maybe she's in a strange place or time where she has no allies to help her, which makes it difficult for her to fight back against Young Xehanort's grip.
You can often find such fanfiction on dedicated fanfiction websites. Some popular ones include Archive of Our Own. Just search for 'Young Xehanort and Eraqus' on the site, and you'll get a list of relevant stories.
A fanfiction might focus on a journey they took together. They could be on a quest for a powerful artifact. Along the way, they face various challenges that test their friendship and their understanding of the Keyblade. For example, they might encounter a group of powerful Heartless and have to combine their skills to defeat them.
Kairi might initially be in shock and fear. Being used as a human shield is a terrifying situation. She could freeze for a moment, not knowing what to do.
Sakura would probably be shocked at first, but then she'd get angry. She'd immediately start to struggle and try to free herself. She might call out for help if she thinks there are allies nearby.
Sora would be furious. He'd immediately set out to find Xemnas and rescue Kairi without hesitation.