Arson might be a stretch. Might depend on if that tent met the definition of "building".
"Building," in addition to its ordinary meaning, specifically includes any dwelling, hotel, commercial structure, automobile, truck, watercraft, aircraft, trailer, sleeping car, railroad car, or other structure or vehicle, or any structure with a valid certificate of occupancy.
It's almost certainly Criminal Mischief 1st.
A person is guilty of criminal mischief in the first degree when, having no right to do so or any reasonable ground to believe that he has such right, he intentionally or wantonly defaces, destroys or damages any property causing pecuniary loss of $1,000 or more.
A creative prosecutor might even be able to pigeonhole it as theft if they wanted a stronger charge, depending on the facts. For instance, if they stayed and watched the show there's an argument to be made they just stole the fireworks.
A person is guilty of theft by unlawful taking or disposition when he unlawfully: (a) Takes or exercises control over movable property of another with intent to deprive him thereof; or (b) Obtains immovable property of another or any interest therein with intent to benefit himself or another not entitled thereto.