Kelisa Roberts, 9, and Raven Carey, 11, aren’t typical children: Their favorite foods are celery, broccoli, and Brussels sprouts. But that they are children was apparent at a recent buffet dinner in their charter school’s cafeteria in Harlem, when both girls scooted past the mounds of hummus and chickpea patties drizzled in horseradish sauce and headed straight for the vegan pizza.

“It tastes like regular pizza,” Raven said with an approving grin.

The buffet was a preview of what lunch will soon look like for a group of students at the Future Leaders Institute, a charter school that serves mostly African Americans from low-income families. Seitan and bok choy will replace hamburgers and pizza as the school joins with Yale University researchers to study the effects of healthy food on student performance. The dinner, served at a fund-raiser for the study, was also a hint of some of the difficulties researchers could face as they try to win over their subjects with “plant-based” menus full of fruits and vegetables.

School lunch is becoming a popular research topic for education and nutrition experts around the country. This month, President Clinton’s foundation launched a joint initiative with food manufactures to improve the nutritional content of school snacks and meals.

The Yale researchers say their study is different. Instead of, say, replacing whole milk American cheese with lowfat, children involved in the study will eat vegan foods such as seitan skewers in chimichurri sauce — which Raven and Kelisa tried later in the evening and said they liked as much as the pizza. Most of the food will be completely new to the students, and the study’s sponsors say they are hopeful that the meals, to be catered by a gourmet Upper East Side restaurant, will be tasty enough to win them over.

“Everybody says it’s hard to get kids to eat this healthy food,” the president of the New York Coalition for Healthy School Lunches, Amie Hamlin, said. “But if it’s really good food we want to see if they’ll want to eat it.” The coalition is one of the study’s sponsors.

The study will include between 50 and 150 children, a smaller group than many other recent school lunch studies, some of which have incorporated whole school districts and have produced mixed findings. The students will join voluntarily with permission from their parents for the spring semester. On school days, the Candle Café on East 79th Street will deliver breakfast, lunch, and two snacks to the school on West 122nd Street, a neighborhood with high rates of obesity and diabetes.

A study of children in Head Start programs released by the city’s health department in April showed that one in four was obese by age 2 and that the rate of obesity increased with age. African-American and Hispanic children were more than twice as likely to be obese as white children, the study showed.

The co-director of the Future Leaders Institute, Marc Waxman, goes to his school’s cafeteria every day for lunch. He said nutrition has long been a concern for administrator.

“I see some kids not eating at all,” he said, “and I see some kids bringing food from home or that they buy on the way that’s not necessarily healthy. … I see other kids wolf down their food and still be hungry. It’s always something we’ve wanted to address, but we didn’t know how to do it.”

The director of research at Yale University’s Rudd Center for Food Policy and Obesity, Marlene Schwartz, is leading the study. She said putting children on a diet is not the idea.

“The point is that there are certain foods kids aren’t getting enough of, and at the top of that list are fruits and vegetables,” she said. “The question is how do you get more fruits and vegetables into them.”

Children generally don’t accept a new food until they have tried it at least five times, according to the study sponsors, who said persistence would be a major part of the effort.

“We will make beautiful, kid-friendly food,” a Candle Café co-owner, Joy Pierson, who invented the chimichurri skewers, said. “But we also have to be persistent. We can’t give up.”

Researchers will monitor the children’s weight, and possibly their blood sugar levels, along with their grades and test scores. They will also measure plate waste — how much food the children throw away — and teach cooking and nutrition classes to the students and their parents. Their hope is that they’ll take newly acquired tastes and eating habits home with them on the weekend.

In Raven and Kelisa’s case, they may be preaching to the converted.

“I think adding a healthy lunch is good,” Raven said. “Most of the days we don’t even have vegetables, it’s just chicken and pizza.”

Kelisa chimed in: “I like my mom’s food. She steams the broccoli.”

As for the other children in the study, Ms. Hamlin said, “We don’t know what the results will be. … But we hope with the right education that’s motivating and with food that’s really delicious, that it will be enough to make a difference.”