What is the ruling on taking metal forks and spoons and plastic cups from the airplane on the pretext of making children happy?

The answer to this question depends on the airline's regulations and conditions:

If the regulations state that passengers own these items, there is no harm in taking them. If the regulations state that they belong to the company, it is not permissible to take them. In case of doubt, the default rule is prohibition, and it is not permissible to take them unless there is evidence indicating the company's permission, based on the hadith: "The wealth of a Muslim is not lawful except with his willing consent." A Muslim must seek what is lawful and avoid doubtful matters, based on the hadith: "Indeed, what is lawful is clear, and what is unlawful is clear, and between them are doubtful matters."