The World Summit for Children, the largest gathering of presidents and prime ministers ever, began Saturday at the United Nations with a public plea for payment of a world debt and a series of private meetings.

"The world has a social debt it has to pay to the poorest," said the first head of state to address the meeting, Chile's President Patricio Aylwin Azocar, at a background briefing.In the world, 40,000 children die every day from causes that could be avoided and prevented, Aylwin said. In Latin America and the Caribbean there are more than 100 million poor people, mostly children, he said.

Canada's Prime Minister Brian Mulroney, one of the initiators of the unprecedented summit, met with Japanese Prime Minister Toshiki Kaifu in a basement conference room at the United Nations.

They were the first world leaders Saturday to hold private, bilateral discussions in the complex.

The summit, witnessed by fewer than 200 children, has strained the capability of the United Nations to handle the more than 70 leaders, including President Bush, Britain's Prime Minister Margaret Thatcher and Czechoslovakia's President Vaclav Havel.

Summit organizers were

stunned and dismayed to learn Saturday that Bush will leave U.N. headquarters Sunday without signing a declaration on children's rights with other leaders. Bush was too busy with a whirlwind of one-on-one meetings.

U.N. Secretary-General Javier Perez de Cuellar, who greeted the arriving dignitaries, said nearly all the officials were "extremely concerned about the difficult situation" in the Persian Gulf.

However, they combined their concern with hopes for the success of the summit, which he described as a "tender occasion."

Nearly 150 motorcades took the dignitaries from their hotels to the U.N. building for a series of bilateral and multilateral meetings and a windup dinner exclusively for heads of state and government Saturday evening.

Participants were asked to be present at 7 a.m. Sunday for a daylong debate.

Had it not been for the Persian Gulf crisis, King Hussein of Jordan and King Fahd of Saudi Arabia would have attended. Except for the Kuwaiti emir and Lebanon's Prime Minister Selim Hoss, all other Middle East leaders stayed home.

King Baudoin of Belgium will be the only monarch at the summit. All Latin American heads of state and government will be pres-ent. African leaders will show up in force, including Uganda's President Yoweri Museveni, Mozambique's Joaquim Chissano and Zimbabwe's Robert Mu-ga-be. Soviet President Mikhail Gorbachev and China's Prime Minister Li Peng first accepted the invitation but later declined in order to deal with domestic problems, U.N. officials said.

"This unique forum will contribute to greater unity of world forces for the sake of a better life and a better future for succeeding generations and will become a landmark in the efforts of states to secure the destinies of human civilization," Gorbachev said in a statement sent to the conclave.

"Coordinated efforts of the world community can provide a political and moral basis for protecting our children from the threat of war and environmental degradation as well as for ensuring their creative development, adequate education and decent living standards," he said.

The U.N. Children's Fund said many of the child fatalities are caused by a lack of medicines that cost only a few U.S. cents each.

Officials of UNICEF estimate the meeting will cost between $3 million and $5 million.

Scores of governments have contributed $3 million to defray costs of the summit. In addition, New York City police would spend $6 million in overtime for police work, city officials said.

Dr. Nafis Sadik, executive director of the U.N. Population Fund, told the briefing, "Up to 20 percent of all pregnancies are unwanted. The first child of a teenage mother is 80 percent more likely to die than the second or third child of a woman aged 20-24."

"Children born less than two years after their older brother or sister are 66 percent more likely to die in infancy," Sadik said. "By expanding effective, modern family planning, infant and child deaths in the Third World would be cut to by 3 to 5 million; perhaps a third of the estimated 13 million such deaths annually."

Diarrhea was the single most fatal disease, causing 4 million deaths annually among children under 5 years of age.

However, Dr. Hiroshi Nakajima, director general of the World Health Organization, said 700,000 children already have been born with AIDS, and estimates 700,000 have been born infected with HIV - or human immunodeficiency virus, which causes AIDS.

He told the briefing that by the year 2000 over 10 million infants and children will have been infected by HIV, and at least 1 million will die of AIDS.

"More than 10 million uninfected children - mostly in sub-Saharan Africa - will be orphaned as their parents die of AIDS," he said.