A treasure hunt! "You mean with clues and secret messages and stuff like that?" asked my son Jeffrey, 8. "All right . . ."

I've been on two treasure hunts recently, at the most unlikely-sounding place: The National Collection of Fine Arts, where the staff really knocks itself out - "We'd practically stand on our heads and whistle, if that would do any good," said one of the volunteer guides - to make the museum visit fund and even a little bit educational for the whole family.

The staff, under the direction of Assistant Curator/Elementary Education Margery Gordon, prepares mimeographed treasure hunt guides, with descriptions of paintings, shapes and colors that the children can search for as they go through the galleries. The first time I went on the hunt, last year, it was with a mother-and-child tour group that I managed for the Recreation Department, the children ranging in age from three to seven. Just a couple of weeks ago, I went again, with Jeffrey.

On the group's bus ride down to the gallery, my partner Marjorie and I showed the children bright triangles and squares and circles and told them we'd be hunting for these shapes in the pictures. At the museum, the children were given their own treasure-hunt sheets and crayons for marking them. The mothers soon decided that it would be better if they held the crayons, because the guards seemed to get a bit nervous at having so many children milling about waving crayons.

A few alarms were accidentally tripped when a child ("There's a triangle in this picture, I see it!") got a bit too excited and brushed against a painting. The children did the best when we sat down in front of a large painting - the bigger the better - and started. And they often found shapes in the most unexpected places. "There ARE SO lines in there!" one little boy insisted as I was pointing out all the circles in one of the portraits. "Look at his shirt collar!"

We started on the second floor - the least interesting from a child's point of view, because the paintings are mostly landscapes and rather dark and somber - and then went up to the third-floor Lincoln Gallery, where there are lots of brightly colored abstracts and a large, round marble sculpture. "It looks just like "a doughnut!" several children said in surprise. With their mothers help they found paintings made all of triangles, all of lines and even one of all the same color. We stayed about 15 minutes.

Our next stop was the Explore Gallery, which the children entered by crawling through a large circle in a bright yellow door. This room, especially designed for children, contains a riot of things to touch, look at, jangle, sit on and even crawl under. The whole effect, especially with so many children, was almost overwhelming.

It was much better when Jeffrey and I went back a couple of weeks ago: There were only two or three other children in the gallery, and we had time to look around and find the items on the treasure hunt, the free-form shapes, the carpet that felt like a desert, the shadows that changed color when a light switch was pushed.

The second floor, page three of the treasure hunt, was his least favorite; but by following the careful directions ("Get off the elevator and turn left") he found the needed pictures containing "earth things like sand and mud, grass and rocks." Well, actually, he didn't exactly find sand - but he stopped in front of "Entrance to the Harbor" by Ranger and said, "There's boat and an ocean, so there must be a beach and sand around somewhere." I figured that was close enough.

The third floor was a delight , because we had to hunt for Indians.

"Find the picture of the Indian man with a feather in his hair and a horse with hands painted on it," said the instructions. Jeff stared intently at the seven painting on one wall, and then: "There it is!" His grin had a little surprise in it. "It's really there," he seemed to be thinking, "and it looks like it's supposed to look."

The last thing we had to find were pumpkins and apples and "a lady who is crying because she is peeling onions." We were both tiring by now, so we asked a guard for a little advice about there to look. He seemed happy to help: "Most people only ask me what time it is," he said as he pointed us in the right direction.

We finished our hour in the gallery with a snack in the very charming cafeteria, "Patent Pending," where Jeff had a hot dog and bottles of wind in favor of dietetic frozen yoghurt. Before leaving we strolled, briefly, in the courtyard to look at the sculptures. It was too cold to linger, but in pretty weather the courtyard's good place to bring a picnic lunch: There are lots of tables and chairs available.

The treasure hunts may be picked up at the front information desk, but if you're going to bring a large group it would be a good idea to call Ms. Gordon (381-6541) to be sure of having enough.

I think I enjoyed the treasure hunt as much as the children - maybe more. Museums are often so overwhelming that I don't know where to look first. The treasure hunt sort of made the gallery manageable. I only wish the staff would make one up for adults.