public class ParentClass
{
   public int Property1 {get; set;}
   public List<Child> {get; set;}
}

public class ChildClass
{
   public int Property1 {get; set;}
   public string Property2 {get; set;}
}

// in my case all data has been assign to parent object properties except List<ChildClass>
List<ParentClass > parent = data

foreach(var parent in parents)
   parent.Children = GetChildren(parent.Id);

parents.ForEach(p => p.Children = GetChildren(parent.Id));

var parent = data.Select(p => new ParentClass{Property1 = p.Property1}).ToList();