It's almost 25 years since Oct. 3, 1955, and oh my, the changes. Mister Greenjeans has turned definitely gray, the "Kangaroo" theme song has been all but swallowed up by a big splashy new song, and Tom Terrific has disappeared altogether.

Only Captain Kangaroo himself seems untouched by the years. He still talks in his soft, calm way. He still has to call down Bunnyrabbit now and then for making those tricky deals with carrots. (He does call Bunnyrabbit "Bun," which must pain Bunnyrabbit no end, because Bunnyrabbit is a serious rabbit.) He still brings his gentle self into the homes of just all the under-7s in the country.

"The problem is getting talent," the Captain otherwise known as Bob Keeshan, said yesterday on a visit to Washington for a silver anniversary party. "It always was. If you go with inferior writings and production, young people will recognize it. They may not speak out, but they know it."

Also, the talent graduates. A 21-year-veteran writer quit to do sitcoms recently. He's got an Emmy alreay. Tom Terrific's animator went into quiz shoes. Many "Kangaroo" creators joined "Sesame Street," which the Captain calls the complement to his show.

"There's no competitive feeling. We need all the good children's TV we can get," he said.

The party took place in a tent behind Decatur House and featured a gorgeous spread of oysters on the halfshell, scallops, shrimp de John, codfish balls and other nautical items as well as the usual meats and cheeses.

Someone thought it was because the Captain is obviously nautical. Someone else said, no, the Captain originally came on as a streetcar conductor in the first show (hence the conductorish cap).

But the Captain himself said they were all wrong. He chose the title just because it sounded nice with Kangaroo. And Kangaroo? Well, that sounds nice with Captain . . .

In any case, the tent was abuzz with anybody who might be interested in children's TV, from legislators to arts administrators to four of the seven FCC commissioners. Plus, of course, a pin-striped platoon from CBS. Another party will be held in New York on the exact anniversary date of "Captain Kangaroo."

It used to cost $12,000 to put on one show. Today the 53-year-old Captain has no idea what it costs, what with the 110 staffers, including a dozen writers and three editors, and the sets and production numbers. It used to be live but now is taped in short segments, giving it a generally punchier look. It is still easygoing, however, compared with "Sesame Street," with its 200-plus staff and educational razzle-dazzle.

This week Bill Cosby joined the show with a five-minute Picturepages bit designed to "enhance the verbal and conceptual skills of preschoolers," as they say in the trade. There are all sorts of other new characters and features, notably John Schuck, the Daddy Warbucks of "Annie," and Tony award winner Priscilla Lopez. A black man and a working woman also appear now and then, along with the old standbys, Mister Moose and Grandfather Clock and Mister Greenjeans.

Greenjeans is Hugh (Lumpy) Brannum, a former bass player with Fred Waring who met Keeshan two years before "Kangaroo." Keeshan was looking for a job. He had been Clarabelle the Clown for "Howdy Doody" and had started some other children's programs. Brannum couldn't get him work, but soon after, Keeshan put all his experience and talents together in a new format, which CBS loved, and he brought Brannum in with him.

Keeshan, who answers to Captain just as comfortably, grew up on Long Island, went to Fordham and joined the Marines in World War II, later going to work for a network in New York. It was there that he met Bob Smith of "Howdy Doody" and was hired as an assistant.

After five years as Clarabelle, he moved on to Corny the Clown and Tinker the Toymaker before unearthing "Kangaroo" in his subconscious. He and his wife still live on Long Island and have three children in their 20s.

He greets the news that he has the longest-running children's program on the networks with utter calm. He's won three Peabodys, an Emmy and a hatful of honorary degrees just by insisting that children deserve to be treated with respect.

He has been accused of failing to deal with the realities of life such as violence and inner cities. He finds the charge a bit trendy.

"We think that real life for 7-year-olds is all about getting along together and valuing friends and having good health habits. There are millions of kids who don't live in the inner cities, after all. The main thing is to make the child feel good about herself. Fred Rogers (of "Misterogers") is doing it. We're trying to motivate young people. We're trying to make them feel they're of value in the world."