The body of a little girl allegedly beaten to death by her adoptive father was turned over to the child's natural mother for burial. Michelle Launders, 26, an insurance company secretary who lives on Long Island, said she regretted giving up the child. "There is not a day that went by without me thinking about her," Launders said. Sources close to the case revealed a grand jury was investigating charges that the man to whom the mother gave the infant six years ago--lawyer Joel Steinberg--was part of a black market baby-selling ring. Steinberg, a Greenwich Village criminal defense attorney, and his common-law wife, Hedda Nussbaum, have been charged with second-degree murder. In an emotional hearing in New York Surrogate's Court, Launders won the right to bury the girl.