The proportion of Hispanic-origin children in the United States living in families with cash income below the government's official poverty line increased to 38.7 percent in 1984, a year of lower poverty rates for most other groups, according to a report by the Congressional Research Service.

The report, prepared at the request of Reps. Harold E. Ford (D-Tenn.), Charles B. Rangel (D-N.Y.) and William B. Richardson (D-N.M.) and released yesterday, said that the number of Hispanic-origin children living in families below the government's official poverty line rose to 2.3 million, 1 full percentage point higher than in 1983. The poverty line for a family of four in 1984 was $10,609 in income in a year or less.

In contrast, from 1983 to 1984, the overall national poverty rate for all persons of all races and origins in the United States fell from 15.3 percent of the population to 14.4 percent, and the rate for all children under 18 from 21.8 to 21 percent.

"Hispanic children were the only children for whom the Census Bureau reported a 1984 rise in poverty rates," the CRS report said.

Moreover, in female-headed families in which the father was not present and where poverty rates for all groups are generally high, the percentage of Hispanic-origin children in poverty rose to 71 percent.

The report found that different subgroups in the Hispanic community had sharply different poverty rates.

"More than half the children of Puerto Rican origin were poor, but only 26 percent of the children of Cuban origin were poor," the report said. For Mexican Americans, the figure was a bit over one-third.

"Contributing to the high rates of poverty among Hispanic children are very low levels of high school completion by their parents, a relatively large number of children per family, an above-average share of children being raised by the mother alone, and a relatively large proportion of children with young parents" whose earning capacity has not yet risen, the report said.

"Whether in a married couple or a female-headed family, most Hispanic children have a family head who lacks a high school diploma."

The report said that "in 1980, more than two-thirds of all poor Hispanic children lived in three states: California, Texas and New York."

The report calculated that government cash welfare and income payments per Hispanic child declined nearly a fourth in purchasing power from 1975 to 1983, as benefits failed to keep pace with inflation.