IT WILL TAKE a long time to catalog the casualties of the massive revision in the nation's social welfare programs that the Senate Finance Committee approved last week. In adopting most of the administration's $9.5 billion cuts, and adding another billion of its own, the committee also rewrote some of the basic terms of Social Security, unemployment compensation, welfare for families, the aged and disabled, Medicaid, Medicare and a host of social services such as child-abuse prevention, day care and home care for the elderly. These cuts are in addition to the $7.9 billion in reductions in Social Security and other retirement programs voted by the Senate later in the week.

The Finance Committee's action is remarkable not only for its sweep, but for its speed and finality. Some of the administration's proposals were not even received until the day the committee approved them. Moreover, under the budget reconciliation process, the committee bill will be unchangeable on the Senate floor. Any changes must come in conference with the House Ways and Means committee that is currently marking up its own version of the bill.

The Senate bill's treatment of one group -- impoverished, homeless children -- will give you an idea of what's involved. There are now about a half-million children in this country being passed from hand to hand in a foster-care system that is a national disgrace. No state can even tell you the number of children abandoned to its care, much less how or where they are. A disturbing number of these children end up being stashed in prisons, mental institutions and hospitals. To reform this abusive -- and costly -- system, Congress (by a unanimous vote in the Senate and with only two dissenting votes in the House) last year adopted legislation to curb uncontrolled use of foster care by states and to provide the services and incentives needed either to return the children to their own homes or to find them permanent homes with adoptive parents. To encourage adoption of these children, most of whom have medical and behavioral problems, adoptive parents were allowed a small grant equal to the welfare payment to which the child would otherwise be entitled and continued coverage of the child under Medicaid. These provisions are very important since almost all families willing to take this kind of child have low or modest incomes, yet their taking the children actually saves money since the cost of the alternative foster or institutional care is much higher.

The Senate bill, though not as harsh as the administration's proposals, would wipe out the adoption entitlement provision and fold the family strengthening services into a single block grant to states together with all the other social services long provided to needy children and others in need. The entire federal amount would be cut by 25 percent from its 1981 level, but the true 1982 cut would be much larger because of inflation and because states will no longer be required to match federal dollars with their own money.

The House Ways and Means subcommittee on public assistance, with the support of some of its most conservative members, last week rejected the administration's child welfare proposals. Members, however, are said to be under strong administration pressure to follow the Senate's lead. We hope that the administration will reconsider what it is doing and, if it won't, that the House will stand fast. If there is one class of citizens that should rate as "truly needy" and deserving of help by virtue of its vulnerability -- not to say sheer personal blamelessness for its plight -- homeless children must surely be it.