It might be an escaped criminal who just happened to come across the boy while he was being tickled. The criminal, seeing an opportunity, kidnaps the boy. The boy's urinating could be a sign of his helplessness and the terrifying situation he has been thrust into by this unexpected kidnapper.
One possible plot could be that a boy is tickled by someone he initially trusts. Then, in a very strange turn of events, he is kidnapped. Maybe the kidnapper uses the act of tickling to gain the boy's trust before leading him away. As for the urinating part, perhaps the boy is so scared during the kidnapping that he loses control of his bladder.
A rival within the police force or someone who wants to discredit the Reagans could also be the kidnapper. They might think that by kidnapping Jamie, they can make the family look bad and gain an advantage in some internal police politics. Maybe they are jealous of the Reagans' reputation and want to bring them down a notch.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.
It could be Vilgax. He has always been after the Omnitrix and kidnapping Ben 10 would be a way for him to get it.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
Maybe it's a group of villains working together. They could be from different factions but united by the idea that Izuku has the potential to become a huge threat to their plans. They might kidnap him to either eliminate him or turn him to their side. Also, it could be a lone, obsessed fan of a villain who wants to prove themselves by bringing Izuku to their idol.