Despite the Senate's unanimous approval this week of a tough new bill cracking down on child pornography and other obscene material, there's room for wondering if Washington isn't promising more than it is prepared to deliver.

To begin with, the pornography bill is attached to an unrelated measure - a highly controversial bill requiring companies to give parents unpaid leave to care for children or ill relatives.Sponsors argue that the pornography bill had to be attached to the parental leave bill in order to get it out of committee, where various opponents had it blocked. But the fact remains that the pornography bill is still tied to a bill that should not be passed because the parental leave measure puts Congress in the position of dictating employee benefits better resolved through bargaining between employees and employers.

The timing of the pornography bill is also awkward, since it goes to the House of Representatives just when Congress is rushing to adjourn in the final weeks of the election campaign.

Still, the need for tougher federal pornography legislation should be beyond dispute. That goes particularly for the most vicious form of pornography - that involving the use of children in obscene films or magazines. A study at Boston City Hospitals shows that the children used in pornography end up with serious mental and emotional problems as a result - problems that may last a lifetime. Likewise, a two-year study by Senate investigators concluded that there's a definite link between viewing child pornography and the sexual abuse of children.

The bill passed by the Senate this week would ban the buying and selling of children for use in child pornography; ban the use of computers to distribute or advertise child pornography; require producers of sexually explicit material to document the ages of persons appearing in the material; toughen penalties against those convicted of selling or possessing with intent to sell child pornography and obscene materials; and give states clear authority to regulate indecent programming on cable and subscription television services.

One of the bill's more disputed provisions would make it illegal to sell or possess obscene material or child pornography on federal lands. Opponents see this as a violation of the Constitution's guarantee of freedom of speech. But the objections are wide of the mark. Surely anything that's illegal to produce or sell should also be illegal to own. And while some states have laws banning the possession of child pornography, there is no such federal statute.

Clearly there should be one. But there may not be as long as the Senate's new pornography bill is tied to an unrelated measure as controversial and inadvisable as the parental leave bill. Congress likely will have to start all over again on pornography legislation after it reconvenes in 1989. Next time, let's do the job right.