With recent advances making it possible soon to save the lives of 20,000 children a day in the developing nations, the United Nations has devoted its final issue of the year to child survival.

A set of six commemoratives, coming out Friday, symbolizes the medical and social breakthroughs in child well-being that can be achieved at relatively low cost and in a relatively short time.

The new issue consists of two stamps for use at U.N. headquarters in New York, two stamps for use at its European headquarters in Geneva and two stamps for use at ancillary offices in Vienna. The issue is accompanied by the 28th in a series of U.N. souvenir cards that reproduces the six stamps.

The World Federation of United Nations Associations, which traditionally accompanies new U.N. issues with cacheted covers and art prints, has for this occasion come up with a moving photograph called "Children Helping Children" by U.N. photographer John Isaac. It shows a solemn young girl carrying her baby brother on her back, and was taken in Bhutan, in the eastern Himalayas.

The 22-cent and 33-cent stamps for use at New York headquarters depict an Asian woman with chopsticks feeding a young boy and another woman breast-feeding her baby. The 50-centime and 1.20-franc stamps in Swiss values show three young girls in school uniforms and a child drinking from a container aided by his mother's helping hand. The 4-schilling and 6-schilling stamps in Austrian currency picture a boy getting a spoonful of food or medicine from his mother and a mother pressing her baby to her breast.

The stamps were designed by artists from Britain, India, Switzerland and the Netherlands and have been produced by a combination of photogravure and engraving and printed by Japan's Government Printing Bureau.

The souvenir card is available for $1.25; with the 22-cent stamp canceled first-day New York, for $1.52; with the 50-centime stamp canceled first-day Geneva, for $1.66; and with the 4-schilling stamp canceled first-day Vienna, for $1.61. There is a 50-cent handling charge for orders less than $2.

This Friday the four annual collection packets become available. These come in separate folders for the New York, Geneva and Vienna series issues. The folders contain a mint single of each category plus descriptive information. The 1985 folder with New York issues is $6.54, with Geneva issues $4.23, and with Vienna issues $4.34. The 1985 Flags installment is $4.22.

Orders or requests for additional information should be sent to the U.N. Postal Administration, Box 5900, Grand Central Station, New York, N.Y. 10017.

In addition to its innovative "courier" covers that bear all six stamps and the same day first-day cancellations from Geneva, Vienna and New York on one cover for $9.95, WFUNA has all six stamps on one cover with only the New York first-day cancellation for $4.

Prints, in a limited edition of 1,000 and hand signed, are available for $38.50 mint or affixed with the low-value stamp and first-day cancellation from New York, Geneva or Vienna.

Cacheted covers with the New York first-day cancellation and the 22-cent stamp are $1.10 each; with the 33-cent stamp $1.20; with both $2.05. Covers with the Geneva first-day cancellation and the 50-centime stamp are $1.50; with the 1.20-franc stamp $1.80; with both $2.70. Vienna covers with the 4-schilling stamp are $1.45; with the 6-schilling $1.55; with both $2.45. Covers with blocks of four and marginal inscription blocks of four are also available.