Children make up the largest block of poor people in the United States, and their poverty rate in 1983 was 22.2 percent, far higher than the 12.7 percent rate for the rest of the population, according to a Congressional Budget Office analysis.

Over the past generation, children have replaced the aged as the most impoverished group in the nation, the study showed.

The analysis, prepared at the request of the Senate subcommittee on education, arts and humanities, gave these figures based on census data:

* 25 percent of all preschool children and 21 percent of school-age children (ages 6 to 17) were in households with income below the government's official poverty line, which was $10,178 for a family of four. The two groups had a combined poverty rate of 22.2 percent.

By contrast, the poverty rate for the elderly was 14.1 percent; for adults under age 65 it was 12.4 percent.

* The number of children under 18 living in poverty was 13.8 million -- about two-fifths of all the poor. They constituted the single biggest group of poor in the country.

* Poverty was far more prevalent among minority children and in households headed by a woman. Among school-age children, for example, the poverty rate was 40 percent for ethnic minorities and 14 percent for nonminorities. In households headed by a woman, the poverty rate for school-age children was 50 percent. In households with two parents or where only the father was present, the poverty rate for school-age children was 12 percent.

The analysis noted that poverty among children, like that for the population as a whole, has been rising steadily since 1979. It said that, contrary to a popular impression, the main cause of the rise is not the increase in female-headed families, but "rather, it is likely the result of the severe, back-to-back recessions of 1980 and 1981-82, the rapid inflation of 1979-80 and reductions in income-maintenance programs."