Hollywood - Eloise, the little girl who "lived" at the Hotel Plaza in New York, is described as the person Henry James would want to study and whom Queen Victoria would recognize as an equal. Perhaps that is why Ethel Barrymore accepted a supporting role in the "Playhouse 90" adaptation of Kay Thompson's book. It will be seen Thursday evening on C. B. S.-TV.

The star is Evelyn Rudie, 7 years old, with just seventy to go to reach Miss Barrymore's lofty status. The Grand Dame will play herself as a resident of the Plaza who helps Eloise with her problems. On occasion the elder turns to the younger for "financial assistance and advice," as Miss Barrymore explained last week during rehearsals.

"I play the horses," she said demurely. "Mental wagers, of course, and Eloise puts me on to the best thing in the fifth at Pimlico." She explained that "the best thing in the fifth" is determined as follows" Miss Barrymore proffers a racing form whereupon Eloise closes her eyes and jabs with a pin. Presto, a winner!

Other tyrannical activities of Eloise, which make her an experience best described as traumatic, are pouring water down the mail chute and strolling with her turtle Skipperdee, on a long ribbon. Nothing unusual about a turtle, you say? This one wears sneakers, has braided ears and eats raising, which Eloise orders from room service.

But Miss Barrymore was serenely taking all this in stride. She pointed out that this "Playhouse 90" role was her first hour-and-a-half live television show. She had not been active of late because of illness.

"Eloise" will mark one of the rare occasions when Miss Barrymore will raise her voice in song. "In all honesty," she said, "I must say that my contribution at that point will consist mainly in speaking the words of the song "How to Raise a Child." Anyway, it's a group song, which will help."

"Eloise" with Evelyn Rudie in the title role and Jack Mullaney as her tutor, will be seen in the C. B. S. -TV Thursday at 9:30 P.M. The story by Kay Thompson is about a little girl who makes life trying for the Plaza Hotel.