Over the hills and far away, Teletubbies come to play. In Teletubbyland, a lush green landscape of undulating hills spotted with clumps of bright flowers, the world is safe and fun — a place to explore and learn through play. We know this because the sun baby, who rises over the set at the beginning of each episode, gurgles, coos and shrieks with pleasure at the adventures of the Teletubbies, four alien yet adorable, toddlerlike beings who live there, cared for and entertained by otherworldly gadgets. Teletubbies, the groundbreaking BBC children’s series that’s prompted both an outcry and a massive consumer craze since its debut last March debut in Britain, is about to arrive in the PBS schedule, April 6. The series is based on the premise — already much-debated in Britain — that very young children are watching television but don’t understand it, so they might as well have a show that’s designed for them.