Try this
SET NOCOUNT ON;
DECLARE @#TempTable TABLE (FirstName VARCHAR(50));
INSERT INTO @#TempTable
SELECT FirstName from #Children
DECLARE @result varchar(8000) = 'ParentID',
@FirstName varchar(50);
WHILE EXISTS (SELECT * FROM @#TempTable)
BEGIN
SELECT TOP 1 @FirstName = FirstName FROM @#TempTable;
SET @result += ' ' + @FirstName;
DELETE @#TempTable Where FirstName = @FirstName;
END
PRINT @result;