THE ALREADY unfortunate plight of foster care children is getting worse. Why? Because of a serious shortage in the number of families willing to harbor them. And those families that are still caring for foster children are finding the job tougher to handle.

In New York City this past spring, a shortage of available foster care homes kept eligible youths sleeping in social service offices overnight. In California last week, foster parents went on strike, refusing to take more children unless they receive government help to cover insurance costs. In April, five children died in a fire in a D.C. foster home; the children had been left in the care of an unauthorized baby sitter who was a fugitive ex-convict. A District official later said that the city needs 200 new foster homes. In Virginia a 14-year-old foster child, missing from a state-regulated group home, was found murdered, and the home was convicted of felony child neglect in connection with her case. A grand jury said state officials knew the company operating the group home, Environments for Human Services, was negligent, but ''that there was a devastating need for such services and that EHS was willing to provide them" and so EHS facilities were kept open almost without review.

Experts cite a number of reasons for the shortage of foster homes. There are more single-parent families and more families in which both parents work. Financial compensation for the foster parents can be as low as $160 a month, a figure that has not kept pace with the rising costs of child care. And the children in foster care, according to foster care officials, now tend to be older, more severely disturbed, more difficult to deal with and therefore more difficult to place. Virginia officials cite some of these factors as reasons for the loss of 30 percent of the state's foster care parents since 1980.

What can be done? The amount of training and counseling support for foster parents can be increased. That also means increasing the number of social service workers assigned to follow foster children, and raising the amount of money the families receive to care for these children every month. The churches that take in foster care children should also be encouraged. These various steps will be more costly to taxpayers. But consider the fact that it costs $1,000 or more per month to keep a foster child in a group home. And consider the cost to society of doing nothing, as troubled children become adults.