About 800 children and adults wandered through the Gothic expanse of Washington Cathedral yesterday, salvaging the spirit -- if not all the planned events -- of a day in honor of children organized by the Washington-based Children's Defense Fund.

The organization had planned a picnic for thousands, complete with sunshine, carnival rides, clowns, jugglers and a marching band, as a way of focusing attention on issues affecting America's 64 million young people. A day-long curtain of rain drove everyone indoors and shortened the schedule of events.

Marian Wright Edelman, president of the childrens' advocacy group, said the celebration's purpose was twofold: outlining national policies and attitudes that are threatening children, and, simply giving children a day to enjoy themselves.

During a service inside the cathedral, she told the audience that Congress and the Reagan administration have misplaced their priorities when they can endorse the spending of $1.6 trillion for "weapons of death" in the next five years, while billions of dollars are stripped from food stamp, Medicaid and educational programs intended to help the nation's children.

"We have to ensure," Edelman said, "that this nation doesn't make children its first victims in times of scarcity."

The Rev. John T. Walker, Episcopal bishop of Washington, challenged the audience to make Washington a model to the country in how it cares for its youth.

Edelman's 7-year-old son Ezra read passages from the Bible, as did several other young children. The Chorale Connection, made up of children attending D.C. public schools, sang. After all, Walker reminded the audience, it was a day for children.

And the children were children: Unable to frolic outdoors, many skipped along the massive Indiana limestone piers that rise from the cathedral floor like trunks of great stone oaks.

Tamara Knowles, 9, craned her neck to watch the playful ascent of an orange balloon, heading for the vaulted cathedral ceiling 100 feet above her pig-tailed head. She would jump, then laugh startled and amused, at the same time by the blast a bursting balloon sent echoing through the cathedral's hush.

"I think this is fun," she said, as her eyes shifted to a party of mimes who silently shouted at each other in exggaerated gestures. "I want to come back again."

Tony Jacobs, 12, was having a good time collecting Childrens' Day buttons. At the end of the day, he had more than a dozen pinned to his sweater.

Dan Smith, 14, said he and a minibus packed with friends from his church came to the cathedral for the service. Despite the soggy weather, he said he was having a good time, "walking around, looking at everybody buried here," referring to his visit to the tomb of President Woodrow Wilson.

Like so many others, Smith had brought his picnic lunch for outdoor consumption, but settled instead for a chunk of the cathedral's marble floor to gulp down a peanut butter sandwich and a handful of sunflower seeds.

Jerry Pritchett was relaxed as he held his two sons in his arms and all three listened to the D.C. Youth Orchestra play a selection from The Sound of Music.

"The kids are mesmerized by it," Pritchett said of his sons, Luke, 3, and Loic, 5. "This is the first time they have heard an actual symphony."

Of the peace that settled over his sons, he nearly whispered: "And this is one of the benefits of this being inside."