They cultivate a lot more than vegetables at Children's Garden

Planting a garden. Growing vegetables and flowers. It's a springtime activity that thousands of city children might dream about but never have the opportunity to do.

But Saturday, April 28, was ''planting day'' at the Children's Garden of the Brooklyn Botanic Garden, and dozens of borough youngsters between the ages of 9 and 17 turned out to plant vegetable and flower seeds in their own assigned garden plots.

During weeks to come, they will tend, water, and weed until they can harvest their crops and pick their blooms. Last year the children reaped 11/2 tons of tomatoes, a quarter ton of broccoli, a half ton of beans, and 17,000 radishes. They also took home plenty of carrots, beets, scallions, squash, eggplants, cucumbers, and corn.

''It is wonderful to see the joy that children take in sharing their flowers and produce with their families,'' says Doris Stone, director of education. If a child doesn't grow a row of his own zinnias or marigolds, he is welcome each week to take home a bouquet of flowers from the annual bed that is cultivated in common.

The Children's Garden of the Brooklyn Botanic Garden began in 1914 as an educational experiment. Since then it has continued to serve as a prototype for other children's gardens throughout the world. It was founded by Ellen Eddy Shaw , who devoted the next three decades to working out a program to teach city children about botany and horticul-ture and to give them the opportunity to practice what they learn in gardens of their own.

Over 15,000 children have cultivated that same patch of city land Miss Shaw laid out, with 4-by-15-foot plots for younger children and 8-by-15-foot plots for older ones. A team of two children works each plot, not only cultivating the ground but, as Miss Shaw intended, learning lessons in self-discipline, respect for the environment, the dignity of labor, generosity, and respect for the rights and property of others.

At the end of each spring and summer session, children are responsible for cleaning the tools they have used and returning them to the instructors for inspection. Because of this good care, some of those hoes, rakes, and cultivators date back to 1914.

To join this gardening venture, a child must love nature, want to learn to take care of a garden, and be able to attend Saturday classes regularly. The fee is $20 per spring or summer session, but scholarships covering that cost are available.

Children in the spring session come to the Botanic Garden in March for three Saturday classes indoors on gardening techniques such as measuring, interpreting garden plans, and labeling. They are placed in groups by age, each supervised by an instructor and a junior instructor. Older children also work in the instruction greenhouses, learning how to plant seeds and transplant seedlings.

After ''planting day,'' they come every Saturday until the end of June to weed, cultivate, plant, hoe, and prune their gardens. Some will take miniclasses on insects or annual flowers; others contribute to the Children's Garden newspaper. Some will take weeds to the dump, mulch plants, deadhead the annual border, or work in the herb garden, the dried flower bed, or with the compost pile.

Children who enroll in the summer session will come two mornings a week, gardening one morning and taking classes or trips to nature centers on the other. General topics covered in the classes include herbs, weeds, pond life, soils, cooking with vegetables, and flower-arranging.

In August the children make dried flower arrangements and paintings, drawings , and collages with horticultural themes to sell at the Harvest Fair in September, the proceeds of which benefit the Children's Garden. At the fair, children lead tours of the gardening area, give cooking demonstrations, and exhibit their produce.

This year about 250 children are expected to participate in the spring and summer sessions. Besides Ms. Stone, the staff includes Betsy Jacobs, children's garden coordinator, and full-time and part-time instructors. Mrs. Jacobs's 15 -year-old daughter, Anita, began coming to the children's garden when she was 9 and has been back each year since.

Summer fellowships for college students with training in horticulture go to American students and to those from other countries, including India, Holland, Taiwan, England, Sweden, and South Africa.

Contributions from the Exxon Corporation, Citibank, L.A.W. Fund, Morgan Guaranty Trust Company, the Institute of Museum Services, Natural Heritage Trust , and High Winds Funds have helped fund the program.