Three-year-old Brian Brown and Nicholas Johnson yesterday climbed up pulleys used to raise cinder blocks, saw what goes on inside a red firebox and could have tried on firemen's tall boots - except that they would have been swallowed up.

Brian and Nicholas were fascinated - and that's the intention of a very special place called the Capital Children's Museum, where a hands-on policy for young visitors encourages them to tug on pulleys and levers, take apart old typewriters, and open things to see what's inside.

The museum opens next week, with hours from 10 a.m. to 3 p.m. Tuesday through Friday and 11 to 3 on Saturdays. But if kids turn up during these hours between now and then, the word is that they won't be turned away. In fact, the facility has been admitting visitors since late July.

Yesterday the Museum announced the coming of Measureland, an outdoor "learning ground" where children cna get to know the metric system by taking their weights on see saw scales and pacing off the kilometers from New York to San Francisco on a topographical map splashed all over the playground.

There will also be Mrs. Holden's wall. Ethel M. Holden is the next-door neighbor to the playground of Capitol Hill's Lovejoy Elementary School at Her blank wall will blossom into a Capital Children's Museum. Mrs. Holden has a lovely, vast, blank wall. and she has given it to Measureland to paint a wall mural.

"I'm happy to be a part of this," said Mrs. Holden yesterday, who was enjoying the ceremonies as much as any 3-year-old.

Her blank wall will blossom into a colorful Hawaiian scene and information tablet on the history of weights and measures.

"Measureland will be the first of its kind in the country," Ann Lewin, director of the Capital Children's Museum, said yesterday. Measureland was her idea and it can be completed within a few months, once $30,000 is raised.

Thomas Sayre, an environmentalist sculptor, and Roy Mason, an architect of the futuristic school, are having as much fun with plans for Measureland as they hope children will have once it is completed.

"There will be footprints in the cushioned asphalt so tha t the youngsters can pace off the distance and make guesses of the distance and then check their predictions like a game said Sayre. "This will be a topographical profile of the United States with in-scale heights for mountain ranges like the Rockies."

Sayre now is working on spinning discs to demonstrate circular measures like clocks, angles, and areas.

Mason, a graduate of Yale University with a master's degree in architecture, has worked on designing playgrounds for retarded children. He speaks of Measureland as a "learning ground."

"It's difficult for a youngster to understand the concept of an angle," Mason explained. "But when a child goes down a slide at an angle, he feels it with his body."

The Capital Children's Museum, formerly called the Center for Inquiry and Discovery, now is housed on the second floor of the Lovejoy Elementary School. It is supported by private funds from foundations and gifts, but the District school system donates the space and utilities.

One of the ideas at the museum is recycling of materials.

"We're always looking for new sources of discarded machines. Then we ask for egg cartoons and paper rolls and hangers," said Lewin.

Old potato-chip boxes are used as building blocks. With a hanger and a piece of string, a child can play "hanger-banger" to feel the vibrations when the string-suspended hanger hits an object as the child holds fingers in its ears. Old inner tubes are cut in designs and glued on pieces of wood to make woolblocks for printing.

After the official opening Tuesday, classes and groups should call ahead for an appointment by dialingthe letters "MET-KIDS."