Dena Hammerstein’s non-profit org may be called Only Make Believe, but it’s been a reality for 13 years — and has just expanded its activities into Washington, D.C.

The charity, which brings interactive theater to hospitalized children, has tapped a starry array of supporters — including Ian McKellen, Rachel Weisz and Jude Law — to augment its teams of three actors who spend six consecutive weeks with the children of a particular hospital unit. Founded in 1999 at a single Gotham hospital, the org has since grown to encompass 50 medical facilities in all five boroughs.

With D.C. hospitals now added to the list, Hammerstein — wife of the late producer-helmer James Hammerstein (Oscar’s son) and a legit producer in her own right — foresees further national expansion for the initiative.

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“It’s such a simple idea, really,” she says, adding that the effort was inspired by her early work as an actress in the U.K. touring special-needs schools. “The most satisfying work I’ve ever done is volunteer work. Every time you come out of a hospital, you feel exhilarated.”

The org’s pro thesps (17 in New York, five in D.C.) perform specially written material, sometimes penned by Hammerstein herself, that’s often an interactive riff on a familiar story or theme, such as a hip-hop redux of “The Emperor’s New Clothes.”

Funding comes entirely from donors and supporters, with a chunk of change drummed up every fall at the company’s annual gala, which last year included the participation of host McKellen and appearances by Alan Rickman, Daniel Radcliffe and “The Book of Mormon.”

This year’s gala is set for Nov. 5.

It takes a substantial amount of behind-the-scenes effort to pull off such a big event with boldface-name talent.”It’s like doing a wedding every year,” Hammerstein says with a laugh.