Despite efforts to control population growth, the worldwide number of children under the age of 15 will increase by half a billion by the year 2000, according to a report released yesterday by the Population Reference Bureau.

Nearly 90 percent of that increase will occur in underdeveloped countries, a disturbing prospect, according to the report.

"The capacity of the poorer countries to improve life for their children will be constrained simply by numbers, particularly the numbers of the dependent young," the report said.

The child population boom in the poor nations of Africa, Asia, Latin America and in the Pacific islands could also fuel further social and political unrest in those regions, the report said.

For example, according to the report: "The unemployment crisis in the developing world [by the year 2000] will be beyond anything yet experienced in the Western World.

"The problems of rapid urbanization will greatly intensify with population growth as many millions of the rural unemployed and underemployed move into the cities of the Third World....

"These two aspects alone, of a growing labor force and increased urbanization, pose the threat of mounting social and political tensions, massive unrest, and spreading conflicts."

In 1975, the latest year for complete figures on the subject, 1.2 billion, or 83 percent, of the world's children, lived in the regions of underdeveloped nations. Seventeen percent, 238 million, lived in Europe and North America that year.

Children under 15 now account for 36 percent of the world's 4 billion overall population. They will make up 32 percent of the total 6.1 billion world population by the year 2000. The proportional decline will result largely from a falling fertility rate -- the number of live births per 1,000 women, aged 15 to 44 years, in a given year.

The PRB, a private, nonprofit, Washington-based research group engaged in population studies, said in its report that living conditions for children in the developing and developed worlds vary widely.

Here are some examples:

The death rate for infants in developing countries is five times as great as the rate for infants born in Europe and North America.

About half the world's 50 million deaths each year are children under 5. Most of those early childhood deaths occur in Africa, Asia and Latin America.

India has one teacher for every 80 children of school age, from 5 years old to 19. The United States has one teacher for every 20 children in the same age group.

However, despite its greater affluence, the developed world also has major problems taking care of its children, the report said. Some examples:

Each year in the United States, nearly 2 million children under 17 are the victims of parental abuse.

Ten to 20 percent of the children in developed countries live in poverty. In many cases they live under conditions of absolute poverty similar to those affecting most of their counterparts in underdeveloped countries.

With growing numbers of single-parent and two-working-parents families, increasing numbers of the children of affluence are suffering from "delibitating stress."

Despite its litany of gloon, the report ended on an optimistic note -- sort of. It said the adult world has the resources to cheaply meet the needs of its children. The only questions are how, and whether, those resources would be used.