Reports of housing discrimination against children have swamped federal officials during the 18 months since it became illegal to bar families from homes and apartments, leading all other complaints of bias, government officials said this week.

About 12,800 discrimination complaints have been filed since March 1989, when the law went into effect, according to Department of Housing and Urban Development officials who attended a conference of the National Fair Housing Alliance in Washington this week.

So far, charges have been filed in nearly 100 instances and several cases have been sent to the Justice Department for trial, they said.

Paul F. Hancock, chief of the department's housing and civil enforcement section, said lawsuits involving bias against families outnumber other types of discrimination litigation in his office. Many other complaints are settled at HUD through conciliation or in cases heard by the department's administrative law judges, according to HUD.

The Fair Housing Amendments Act of 1988 added families with children and handicapped persons to the groups of Americans who are protected against discrimination.

Families now have the same rights to buy or rent all types of housing, from mobile homes to single-family houses, as people without children. The law also covers vacant land that is purchased or leased for building or placing homes on it.

Colleen Fisher, head of the government relations department of the National Apartment Association, a trade group for apartment owners and management companies, said the organization is working hard to educate its members on the law. Although the group opposed some aspects of the law, she said, "Our members are trying to look at this from a positive perspective." There still is uncertainty among landlords as to "what is considered to be a reasonable occupancy standard. Problems that arise usually revolve around what property managers can do and cannot do in terms of day-to-day management."

The Justice Department's first case under the new law was filed against the owners of a New Jersey apartment complex who required a single parent with one child to rent a two-bedroom apartment while permitting two adults to rent a one-bedroom unit, Hancock said. The department won the case and a $33,000 award that went to the single mother in the case and the fair housing organization that helped investigate the complaint, he said.

Hancock said that in some instances landlords have given authorities creative excuses for refusing or limiting the number of children allowed in a home. He said one Midwestern property owner specified that only two children per family could be allowed because of limited sewer capacity, and another landlord said the local school system could not take any more children.

Reports of housing bias go first to the Department of Housing and Urban Development, where officials try to help both sides reach an agreement. But if this does not work, people who believe they have been discriminated against can ask for a trial handled either by an administrative law judge at HUD or in court by Justice Department attorneys.

Much misinformation, misunderstanding and hostility surrounds the new law, said Lisa Mihaly of the Children's Defense Fund, a national child advocacy organization. Opponents argue that renting to families with children will drive up the costs of housing, or will hurt business, she said, adding, "Anything you have heard about race cases, you will hear about these cases."

In reality, Mihaly said, the new law is "exceedingly simple." To be protected, "all you have to do is be a person over the age of 18 with legal custody of a person under 18," she said. The family can be a traditional one or a foster family. The law also protects pregnant women who do not yet have children, and people in the process of adopting children.

Despite the law, Mihaly said there still are numerous communities throughout the country where newspaper ads say "No pets, no kids."

"That's discrimination against families," she said. One ad even said, "No little monsters," she said.

Housing discrimination against children is an entrenched practice, experts said. A 1980 HUD study of about 80,000 housing units found that landlords for one-fourth of them had "we-don't-take-children policies and owners of another half had policies that essentially kept families out" by more subtle means.

Mihaly said these rules range from renting to families "only if their children are under a certain age or only if they're willing to live in the unit next to the washing machine."

More recent state and local studies have found these practices are still prevalent, Mihaly said, indicating that "left to their own devises, landlords will willingly exclude large portions of the population from the housing market."

Cynthia Harvey, administrator of the Howard County office of human rights and a former human relations worker for Prince George's County, said she has seen repeated instances of bias against families with children. She said families often are told they must accept ground-floor or second-floor apartments because their children are noisy and will disturb tenants who live below them.

Leases have been voided because of children's noise while adults who have "noisy parties" are warned but not evicted, she said. Families often are required to rent more space, with a woman and two children told they must take a three-bedroom apartment, she said.

"When a complaint comes in, we look to see if the people with no children have been evicted for the same complaint," she said.

Mihaly said tactics used to discourage families with children include extra charges. She said that some landlords will say, "'We'll be happy to rent to you but your security deposit will be $500 instead of $200.'"

Landlords also are not allowed to discourage a family with children from moving in, "just as you can't discourage a family of color, you also can't discourage a family with children from moving into a building without a playground," Mihaly said. "That's their choice."

The most important, most controversial and most difficult of the protections in the legislation is the provision that permits operators of "bona fide older persons' housing" to bar children. Much of the legal action so far has centered on the question of whether property owners are trying to circumvent the law by saying the housing is for older persons even if it does not meet the requirements, Mihaly said.

Housing exempted from the law includes complexes where all of the residents are 62 or older and developments where at least 80 percent of the units have at least one occupant who is 55 or older and provides services for older people.