It should also be a good deal of fun for toddlers whose minds have not yet shriveled into orthodoxy.The visual style is charmingly conventional, as gently reassuring as that of a Donald Duck cartoon, sometimes as romantically pretty as an old Silly Symphony. Guard duty was left largely to a dim-witted vulture who liked to yell "one o'clock and all's well" when it was often three o'clock and everything was terrible.An all-animal/bird version of Robin Hood? Good grief! But then, why not?The legend about the bandit of Sherwood Forest has survived more than a dozen screen adaptations, including "Son of Robin Hood" (1959), which was really about a daughter, and 165 half-hour television films.Walt Disney's multigenus, animated film version, which opened yesterday at Radio City Music Hall, testifies to the legend's elasticity and durability. Nottingham Castle was run by incompetents. Once upon a time in Sherwood Forest there lived a fox named Robin Hood who, with his good friend, a sportive brown bear named Little John, stole from the rich to give to the poor.Nearby Nottingham was occupied by elephants, rhinos, hippos and snakes, who were the escorts, advisers and chums of the evil Prince John, a scrawny lion whose crown was too big and who, in moments of national crisis, sucked his thumb and pulled his ear lobe.