SOME typecasts are impossible to shake. Just ask Christopher Atkins. It has been nearly 15 years since he was seen loping through the palms of Blue Lagoon with a teenage Brooke Shields
. But, he says, people still perceive him "with curly hair, running on the beach in a diaper." Grateful for steady work in what he describes as 14 "little pictures" over the years (and as Linda Gray's younger love interest on Dallas in 1983), Atkins, 33, says his greatest satisfaction is at home in the San Fernando Valley with his wife, former model Lynne Barron, and their kids Grant, 8, and Brittney, 7. "I really did drop out to become a dad," says Atkins, a self-described "old fart," who drives a station wagon crammed with surfboards and soccer gear. "It's so much fun having the little knuckleheads around."