The parents of a 10-year-old boy who died in January after falling into an icy pond in Montgomery County have sued several real estate and construction-related companies, asserting that they failed to keep a mandatory fence around the pond that would have kept the child out. “Had the defendants acted properly, D’Angelo McMullen would not have died,” the parents’ attorney, Andrew G. Slutkin, wrote in a complaint filed in Montgomery Circuit Court on Wednesday.