public class Source
{
    public Tbl_Child Child { get; set; }

    public string SourceName { get; set; }
}

public class Tbl_Child
{
    public int ID_Child { get; set; }
    public string ChildName { get; set; }
}

public class Destination
{
    public int FK_Child { get; set; }
    public string ChildName { get; set; }
    public string SourceName { get; set; }
}

static void Main(string[] args)
{
    var src = new Source()
        {
            Child = new Tbl_Child()
                {
                    ChildName = "ch",
                    ID_Child = 1
                },
            SourceName = "src"
        };

    AutoMapper.Mapper.CreateMap<Source, Destination>();
    var dest = AutoMapper.Mapper.Map<Source, Destination>(src);

    Console.ReadKey();
}

Mapper.CreateMap<Source, Destination>()
.ForMember(d => d.FK_Child, o => o.MapFrom(s => s.Child.ID_Child));

Mapper.CreateMap<Source, Destination>()
.ForMember(d => d.FK_Child, o => o.MapFrom(s => s.Child.ID_Child))
.ForMember(d => d.ChildName, o => o.MapFrom(s => s.Child.ChildName));