Another theme could be the passing on of values. Harry and Ginny would want to instill in their kids the values they hold dear, like courage and loyalty. This can be shown through various interactions and situations in the fanfiction.
It's possible that when they meet their kids, they realize that their family has a special destiny in the wizarding world. Their kids could be the key to uniting different magical communities or solving an ancient mystery. This could lead to a series of adventures where Harry and Ginny, along with their kids, embark on a journey to fulfill this destiny.
It could be that you encountered the kids during a community gathering. Maybe you were volunteering and they came up to you. Another possibility is that you met them at a friend's house.
A young scientist would be a great main character. They could be curious about what the future holds for their research and accidentally stumble upon a way to travel to the future. Their scientific knowledge would help them understand the new technologies and concepts in the future world.
In this fanfiction, it could be that the future has a new Dark Lord. The Marauders are brought forward in time to stop this new threat. They face challenges like understanding the new forms of magic and technology that the future Dark Lord has harnessed. They also have to deal with the fact that the future wizarding world has different social and political structures. They might start by infiltrating the Dark Lord's lair using their Marauder's Map - like skills, but find that the map doesn't work as well in the future, leading to all sorts of interesting and dangerous situations.
Their kids' personalities could be a mix of Marco and Star. They might inherit Star's boldness and Marco's determination. So they would be fearless when facing challenges and not give up easily.
Future Loki might interact with the Avengers in a cautious way. He knows their past opinions of him and tries to prove himself. He could start by sharing small bits of useful information. For example, he might tell Iron Man about a weakness in a new enemy's technology.
I think Uraraka would be very excited. She has always been a big supporter of Deku. Meeting Future Deku would give her a chance to see how far he has come, and she might also get some inspiration for her own hero journey. Plus, she's a friendly and emotional character, so she'd likely be thrilled at the opportunity.
Definitely Captain America. Their relationship has always been complex and full of different ideologies. Meeting future Tony would be a big moment for Steve as he would see how Tony has changed or what new ideas he brings. Also, Thor would likely interact with him. Thor, with his Asgardian knowledge and power, would be interested in the future technology Tony might bring. And of course, Black Widow. She's the master of espionage and would be curious about Tony's motives and what he's hiding from his future.