The percentage of American children whose families have incomes below the poverty line has increased to 22.2 percent, according to a congressional study, while true government spending for those children has dropped by $290 each since 1976.

The study by the Congressional Research Service and Congressional Budget Office challenges the popular theory that poverty among children has increased despite a substantial infusion of federal funds.

Rather than increasing over the past decade, total per-child spending for cash welfare payments, Social Security and unemployment benefits declined 6 percent overall from 1973 to 1983 after taking inflation into account, the study said.

The nation's poverty rate for children under 18 fell to its lowest point, at 13.8 percent, in 1969 after a decade of decline. The rate has risen steadily since then, the study showed.

As both the population and inflation rose in the 1970s, government spending for programs to aid children failed to keep pace. Cash and food-stamp benefits available per poor child from various programs, as measured in constant 1983 dollars, actually dropped from a high of $1,446 in 1976 to $1,156 in 1983. These calculations do not include Medicaid outlays, which rose. Some economists believe Medicaid should not be counted as income, since it is not available for daily living costs.

The report said that from 1971 to 1985, states failed to raise welfare benefits sufficiently to keep pace with inflation and, as a result, the purchasing power of combined welfare and food-stamp benefits for families without other income dropped by about 22 percent.

The rise in single-parent families since the early 1970s is a major cause of the increase in poverty rates, the study said.

The study found that half of all poor children live in female-headed, one-parent families.

Nearly half of all black children, a third of all Hispanic children and17 percent of white children lived in families with incomes below the poverty line in 1983. The government's definition of poverty varies by the size of the family. The level was $7,938 a year for a family of three in 1983, but ranged from $5,061 for a single person to $20,310 for a family of nine or more.

In female-headed white families, the poverty rate for children was 47.6 percent. It was 68.5 percent in black female-headed families and 70.5 percent in such Hispanic families

The rates are higher where the mother never has married.

"For children in black, single, female-headed families where the mother is under 30 and did not complete high school, the poverty rate is 92.8 percent," the subcommittee noted.

The report also found that more than 2.5 million of the 13.8 million children below the poverty line lived in families where at least one person had a full-time job. This "belies the widespread view that a full-time job throughout the year is near-insurance against poverty," the subcommittee report said.

It said that two-thirds of the children below the poverty line during a 15-year period are poor for no more than four years, but at least one out of seven stays poor for at least 10 of the 15 years, or most of his childhood. Such children are 90 percent black, often live in the South and in rural areas, and usually lack a father in the home.

The official 22.2 percent figure for child poverty is based on cash income only, and the study said that if the value of such noncash benefits as food stamps and Medicaid were counted the poverty ratio for children would drop to between15.9 percent and 19.6 percent, depending on how the value of the noncash benefits is calculated.