Saccharin "probably" causes some bladder cancers in males and should be denied to the millions of children who now drink more and more beverages containing it, the federal food and drug chief and a panel of scientists warned yesterday.

The evidence on human bladder cancer is still inconclusive these witnesses told a Senate health subcommittee. But all said "prudent" parents should keep diet soft drinks, which contain saacharin, away from their youngsters. The only exception, said Food and Drug Commissioner Donald Kennedy, would be to allow a small amount to diabetic or obese youngsters who otherwise might consume sugary drinks.

Kennedy and four members of a National Academy of Sciences food safety panel testified before some of the Human Resources Committee senators who must decide what to do after May 23, when a congressional moratorium on any federal saccharin ban expires.

The evidence against saccharin, though sometimes conflicting, has been building up, most of the scientists said.

What is known now, they agreed, is that:

Saccharin is a "weak" carcinogen, causing bladder cancers only in small numbers of animals. But it is also a cancer "promoter" that can help make other chemicals cause cancers.

The evidence on human cancers is still "equivocal," in part because it takes studies of such large numbers of persons to find any cancers that occur in just a few, even though that "few" might amount to thousands in the whole population.

The evidence that indicates saccharin may cause human cancers indicates that it causes them mainly in men.

The sensible course today is to deny continue using it in large amounts throughout life. Many children are consuming "frightening" amounts-and also being exposed to other causes of cancer, among them, cigarettes-said Dr. Emmanuel Farber of the University of Toronto.

Until this week, Kennedy, testified, he had known of just three apparently adequate studies of human bladder cancer-two unable to find evidence of saccharin-caused cancers and one that indicated saccharin did increase a man's risk of bladder cancer by 60 percent.

Only Tuesday, he said, he learned that the author of one of the first two studies, Dr. Ernest Wynder of New York, told the National Cancer Institute in November 1977 that his latest studies had found a doubled cancer risk in saccharin-using men. Kennedy said he was "surprised" that he had not been told before of this "new" evidence, and it had not been available to the National Academy of Sciences panel.

But Wynder said in an interview that his 1977 progress report was only that, not a conclusion. He said he wrote the cancer institute then that the work had many inconsistencies and needed more study.

He indicated strongly that his further analysis, to be made public in about a month, may not confirm the 1977 indications.

National Cancer Institute spokesman Paul Van Nevel said Wynder's findings were known to scientists on the science academy panel, and some members spoke to Wynder about them.

What the Food and Drug Administration must do after May 23, Kennedy said, is again propose - as it proposed in 1977 - that accharin be removed from foods and drinks and limited to use in table-top sweeteners. In this way, he said, the obese or diabetic, who felt they needed it to help them avoid sugar, could buy it.

But the restrictions could not be immediate, he said, since law and FDA rules would allow at least 15 months for comments and hearings, and Congress would have "ample time" to take any further action it wanted. CAPTION: Picture, DONALD KENNEDY . . . renews call for ban