One of the most urgent needs of the entire third world is to achieve what United Nations officials call a ''child survival revolution'' - fighting the ancient scourge of high mortality rates, especially among babies less than one year old.
Today comes some encouraging news that such a revolution might have begun, though the experts warn at once that much, much more remains to be done.
In a report entitled ''The State of the World's Children 1985'' and released in London, the UN Children's Fund (UNICEF) says:
* An opportunity now exists ''to save the lives of approximately 7 million children a year, and to protect the normal development of many millions more, at a cost which certainly does not exceed a fraction of 1 percent of the world's gross national product.''
* The lives of half a million children were saved in 1984 by using a simple, inexpensive method to combat dehydration, which is the single biggest cause of death among third-world infants.
* Lowering infant death rates does not add to the third world's already severe overcrowding problems but actually helps reduce them. UNICEF's executive director, James P. Grant, argues that there will be no ''surge in world population'' if more children live:
''All the evidence suggests that a reduction in the number of child deaths would also help bring about a greater reduction in the number of child births,'' he says.
The greater the chance of a child surviving, Mr. Grant says, ''the less the parents need to insure against loss by bearing more children than they actually want.'' Parents may space their children more widely, and educated women are more interested in family planning - two more reasons, UNICEF says, why overall birthrates will go down.
The new report even prints a diagram claiming that if infant mortality is cut in half in the next decade, world population would reach only 8.5 billion by the year 2100 instead of 10.5 billion as it says other UN figures indicate.
Other UN officials contacted in New York add a note of caution. The UN Fund for Population Activities (UNFPA) agrees that lower infant mortality rates do help reduce total birthrates, but it points out that other factors are also extremely important - raising the status of women, for instance.
''Certainly the experience in Kerala in India, and in Sri Lanka, has shown that falling child mortality rates were followed by lower population growth rates,'' says Jyoti Singh, a senior UNFPA official. ''But many other things also have to happen: better health care for mothers and children, the ability of women to find work outside the home, women's literacy, and so on.''
Officials say the 10.5 billion figure used in the UNICEF chart is the most optimistic of three variants cited by UNFPA: Given more pessimistic scenarios of progress, world population could reach 14 billion, even if progress is made in lowering infant mortality.
Nonetheless, UNICEF is encouraged that a significant start has been made toward allowing many more third-world children to live longer, and it sees a number of added benefits from the trend.
The fundamental issue is not medical or technical - it is one of perception and understanding, many UN officials say.
''We are talking about bringing about a mass ripening for change, about opening people's mental boxes in which habitual acceptance of a low level of health and a high rate of death among their children is locked up as socially or fatefully decreed inevitabilities,'' says UNICEF official Tarzie Vittachi.
''We are talking about transforming deeply inlaid attitudes and practices and helping people to see over the hill.
''What needs to be done then is to present new knowledge to people as a way to empower themselves to change their perception of 'normality' or 'inevi-tability.' ''
Tony Hewett, another UNICEF official, says that one of the most urgent development needs in the third world is spreading the word that change is possible if people know where to go and what to do to find it.
''This is a time in history when the richer countries are somewhat disillu-sioned with 'development,' '' he said in an interview in London.
''I know people have responded to the African famine, but recession and other constraints have generally acted to hold development budgets down and to reduce soft-loan World Bank lending,'' Mr. Hewett said.
''Now we have news of progress toward saving children's lives on a wide scale , at low cost.''
The UNICEF report points to four elements in what it sees as its potential ''revolution'':
* Teaching mothers to monitor the growth and feeding of their infants. Using a simple 10-cent growth chart could help eradicate half of all child malnutrition, UNICEF says.
* Encouraging breast-feeding of infants whenever possible as the best nutrition a baby can have in its first six months of life.
* Restoring salts and fluids to children suffering from diarrhea by using 10 -cent packets of salt and sugar or homemade mixtures of the two. (UNICEF says it has distributed 65 million of these packets to 78 countries so far.)
Emphasizing the do-it-yourself aspect of these methods, UNICEF says they could be spread to half of the world's families in the next five years, saving the lives of about 2 million children every year.
* An expanded program of immunization against six common diseases that the UN says kill or disable 10 million children a year.
UNICEF cites examples of new activity to lower child mortality in Colombia, Brazil, Algeria, Indonesia, Pakistan, Turkey, and elsewhere.
Mr. Hewett singles out a plan by President Belisario Betancur Cuartas of Colombia, due to be announced today, to reduce the national infant mortality rate from 57 per thousand to 40 per thousand in five years.
For comparison, the infant mortality rate in the United States was 10.9 per thousand in 1982.
In Britain the rate was about the same, but in Ethiopia it was 146 per thousand and in Sierra Leone, 206.
UNICEF works through local communities, Mr. Hewett emphasized.
''Development,'' he said, ''is a process going on inside each one of us. We all strive for self-realization. . . . So do people in the third world - and they want to help their families by saving the lives of their children.
''We can help.''