Some, like the mayor, saw it as a hopeful sign, a step in the direction of revitalizing an entire neighborhood. Certainly driving up H Street NE made it easy for anybody to see "that the economy of this street is sagging," he said.

"A new spirit, a new life," Marion Barry continued, giving the Capital Children's Museum the city's official blessing. "It shows you what people can do when there is a will to do it."

And so it did, for some 1,000 invited guests there last night to look over the museum's new home.

"A community is only as strong as its children," said Barry, addressing them from a balcony, colorfully devised as a prop in the inaugural exhibit, titled "Mexico."

Earlier in the day, First Lady Rosalynn Carter had stood in the midst of the museum's simulated Mexican town square. "I feel that I am on the first stop of my three-day visit to Mexico," she said, referring to the state visit she and President Carter begin today.

At the morning ceremonies, there had been an opportunity for her to brush up her Spanish with children from the Mexican Embassy staff. Besides conversing with them, Mrs. Carter led the youngsters in a Spanish rendition of a "happy birthday" greeting to Mexican Ambassador Hugo B. Margain, who also attended the opening.

It will be mid-March before the Capital Children's Museum officially opens to the public in its new permanent headquarters within a few blocks of Union Station.

It's a museum site with special opportunities and problems -- the former convent and nursing home of the Little Sisters of the Poor, with "somewhere around 135 rooms -- everybody has a different count," a spokesman said yesterday.

With a $1.7-million grant from the Department of Housing and Urban Development, the museum trustees bought the convent, which covers the block bounded by H, I, Second and Third streets NE. In its first 1 1/2 years of existence, the Capital Children's Museum arranged exhibits in extra space in the Lovejoy public school. These exhibits remain open until the final shift to the new headquarters.

Yesterday Mrs. Carter, with daughter Amy at her side, toured a town square with a fountain, a farmyard with functioning well and a functioning black goat (as yet unnamed), and a general store stocked with Mexican products and work of Mexican craft artisans, including huge papiermache figures suspended above.

Amy and the other children settled for wooden recorder-like instruments and piped their way around the exhibits.

In her remarks, Mrs. Carter spoke warmly, without notes, of her family's close ties to Mexico and of how she and her husband -- then a young naval officer stationed at San Diego -- used to drive over the border to Mexico "every single weekend."

Mrs. Carter said she was looking forward to renewing her friendship with Carmen de Lopez Portillo, the first lady of Mexico. The two participated in a series of exchange meetings and tours on both sides of the Rio Grande while their husbands were meeting in November 1977.

As Ann White Lewin, the sparkplug director of the Capital Children's Museum, tells it, the Mexican exhibit, with its explosion of color and crafts, became the opening exhibit when the Los Angeles Folk and Craft Museum proposed a loan exhibit in Washington

Then the Mexican government pitched in with offers of help.

"Jill Vexler, our curator, toured Mexico in a car, and they offered us the whole country," said Children's Museum staffer Paula Silvert.

Mexico, at least, did offer pieces of its country -- actual Mexico City street markers, a turn-of-the-century ironscrolled bench from the governor of Oaxaca, tools, utensils and traditional crafts and textiles.

Museum director Lewin noted that the opening Mexican exhibit will have even more "hands-on" activities than earlier shows. Youngsters are encouraged to mount the wooden horses -- as Amy Carter did yesterday.

The grant was made from the discretionary funds of HUD Secretary Patricia Roberts Harris.

Meanwhile, the museum has been singled out as a model of how federal grants can revive parts of urban centers that are located beyond the downtown center.

Revitalization, in fact, according to Nan Powell, vice chairman of the museum, was "exactly why we were able to get the funds" -- $1 million of which are matching funds; $700,000 must be raised locally by Sept. 30, 1980.

Some guests last night took Metro to Union Station, where they were met by shuttle buses from the city's department of recreation. The distance was not great, but for young patrons coming from around the city via public transportation, access lemains a problem.

"There is no easy access," said Powell, "but we envision a yellow brick road someday. It all takes time."