A deficiency of one of the most common elements required in a healthy diet - iodine - has condemned millions of children to cretinism, tens of millions to mental retard- ation and hundreds of millions to subtler degrees of mental and physical impairment in the past few decades.

Most industrialized nations, including the United States, have known for most of this century that adding iodine to salt - at a cost of about 5 cents per person per year - eradicates this tragedy, but in the developing world it has been allowed to continue.Fighting this kind of waste was one of the 27 specific goals of the 1990 World Summit for Children aimed at ending the senseless suffering and death of children caused by ignorance and greed worldwide.

The good news is that, since 1990, 100 of the developing nations, with more than 90 percent of the developing world's children, are making significant progress toward the goals set four years ago, according to the 1995 State of the World's Children report.

Those goals include a one-third reduction in child deaths, halving of child malnutrition, immunization levels of 90 percent, control of major childhood diseases, eradication of polio, elimination of micronutrient deficiencies, better education and clean water.

Working together, the governments represented at the summit have taken action that means 2.5 million fewer children will die in 1996 than in 1990 and tens of millions of youngsters will be spared the pain of retardation and physical impairments.

The countries that have made children's well-being a primary goal should be commended. The future of the world lies with the children, and unless long-term investment is made in their health and happiness, the world is in for a continuation of its long-term problems.

The challenge now is to continue putting children at the top of government priority lists, and the United States should lead the way.

The 1995 Summit for Children in Copenhagen, Denmark, should not spend too much time patting itself on the back for what has been accomplished but must renew a commitment to continue efforts to meet all the goals set four years ago and look for new ways to improve life for future generations.