raywest

Answer: It's never explained. He likely found them in the men's changing room, borrowed them, stole them, or they were provided by the hotel.

raywest ★

Answer: The swim trunks had The Plaza crest logo on them. They must be complementary for use at the hotel (like a robe, etc).

Answer: The credit card company was aware that an unauthorized person was using the card, which someone in the credit card system could misinterpreted as being "stolen." The police needed that information to track when, where, and how the card was being used to locate Kevin. It was assumed that Kevin had the card, but someone else could have gotten hold of it.

raywest ★

Answer: They did not know where Kevin got off the airplane (or how far away he may have ended up). There were numerous flights to various cities and they initially wanted to know if Kevin used the card and, if so, what city (maybe even country!). This would give police an idea of where to start looking; it was a technique to narrow down the possibilities. If one's credit card is "missing" or in the hands of an unauthorized person (such as Kevin being in possession of the card), it should be reported as stolen (or missing) so that the credit card company is on the alert for an unauthorized user. If this missing card is presented to a merchant (or ATM for cash withdrawals), the person will be unable to use it to make any purchase or continue charging items. The credit card company will know if/when someone tries to use the card and can then notify police of the exact location; the police can investigate from that point. Reporting the card "stolen" as soon as possible (as soon as one realises it is missing) helps protect the card's owner from being held monetarily responsible for whatever unauthorized charges are made over $50; the credit card company will consider the card's owner to be limited to a maximum of $50.

KeyZOid

Answer: Harry and Marv go after Kevin probably to even the score for a kid having outsmarted them once before. They tell him what their plans are because they're not particularly bright, believe it doesn't matter, and simply because it serves as a plot device. Kevin has to somehow learn their plan, otherwise he could not thwart them.

raywest ★

Answer: Marv and Harry say in one of the scenes that Kevin is not at his own home meaning "he doesn't have a house full of goodies" to hurt them with (quoted from the scene). So they felt like they were safe from any havoc because he didn't know where he was, of course they underestimated him.