BIL KEANE doesn't have to look any further than his own drawing board for inspiration. Here two of his eight grandchildren--Matilda, a California girl, and her cousin Oliver from Tucson, Arizona--visit Granddad to imprint their personal approval on his cartoon work. Readers of 1,500 newspapers have also been approving of FAMILY CIRCUS since 1960. "Retire?" says Bil. "Don't be foolish. Why quit a job that is so much fun, especially when I have such lovable assistants!"