“Ant-Man” star Paul Rudd may play the tiniest of superheroes, but on Tuesday night the actor showed he has a giant-sized heart.

Rudd surprised an auditorium full of kids from a pair of charities in a special New York Daily News- and Disney Studios-sponsored special screening of the latest Marvel Studios superhero film, days before its release to the public.

“It’s genuine, it’s real enthusiasm, I can hang with kids for hours and hours,” the 46-year-old actor told The News after exiting the packed auditorium at Manhattan’s AMC Empire movie theater to cheers and applause from the 300 in the audience.

Nearly half of them were from Big Brothers Big Sisters of New York, an organization that provides mentors for kids in the city who face poverty or other adversity.

“I was this surprised (to see Rudd),” said 10-year-old Tariq Howell, a little brother with Big Brothers Big Sisters, as he held his arms as wide apart as he could.

In “Ant-Man,” Rudd plays a reformed thief under the guidance of a retired superhero (Michael Douglas), who helps him to reach his potential — a theme to which many in the audience could relate.

The other charity, SAY, helps kids who stutter find their voice through after-school programs, a summer camp and speech therapy. It’s a cause particularly important to Rudd, who discovered the organization’s work while researching stuttering as he prepared to play a person who stutters in the 2006 Broadway play, “Three Days of Rain.” The New York-based actor joined the board a year later.

“I know a lot of these kids for years, so it was really fun to stand up there while they were sitting there with their three-D glasses,” added Rudd who has volunteered at SAY for nearly a decade.

He got to know one kid in particular very well Tuesday, anointing 8-year-old SAY member and superhero fan Kaiden Roman as an official sidekick for the screening. Before the heroic pair entered the theater, they shared a 10-minute conversation backstage after Rudd surprised him in the hallway.

Kaiden’s mom told The News before the screening that her son’s three-months being mentored at the charity have helped the Queens elementary school student pull out of his shell. He had been afraid to talk to classmates because of his stuttering.

“I’m very excited,” Kaiden, decked out in an Ant-Man hoodie, said. “I love (the hero) because he can communicate with ants.”

For his partner in crime-fighting, the experience went even better than it could have been scripted.

“It was the first time I could have a screening for kids for one of the movies I've done,” said Rudd, whose resume consists of a lot of R-rated comedies and indies. “Most of them would not be appropriate in any way shape or form.”

With Peter Sblendorio