public class DocProfile : Profile
    {
        protected override void Configure()
        {
            Mapper.CreateMap<Folder, FolderModel>()
                .IgnoreAllNonExisting()
            .ForMember(right => right.Children, x => x.ResolveUsing<FolderChildrenResolver>());

            Mapper.CreateMap<Document, DocumentModel>()
                .IgnoreAllNonExisting();

            Mapper.CreateMap<TreeNode, TreeNodeModel>()
                .IgnoreAllNonExisting();

        }
    }

public class FolderChildrenResolver : ValueResolver<Folder, List<TreeNodeModel>>
    {
        protected override List<TreeNodeModel> ResolveCore(Folder source)
        {
            var list = new List<TreeNodeModel>();

            if (source.Children == null || source.Children.Count == 0)
                return list;

            foreach (var child in source.Children)
            {
                if (child.IsDocument)
                    list.Add(child.MapTo<DocumentModel>());
                else
                    list.Add(child.MapTo<FolderModel>());
            }

            return list;
        }
    }