How do I make this code loop 7 times and go to the next row?

I've been learning SQL by myself, and I'm stumped on this one.

In this table I'm working on, I have a persons first name and last name as a column, and I'm using a user defined function to concat the two columns to get their full name.

What I have works for one column, but I don't know how to loop it. Here's my code.

CREATE FUNCTION dbo.fullName(
	@firstName VARCHAR(10),
	@lastName VARCHAR(10))
RETURNS VARCHAR(20)
AS 
BEGIN
	DECLARE @fullName VARCHAR(20)
	SELECT @fullName = CONCAT(firstName, ' ', lastName) FROM stray_kids
	RETURN @fullName;	
END

Then when I call it with SELECT lastName, firstName,dbo.fullName(firstName, lastName) FROM stray_kids
It comes up with the image I added, which is great for the first row and then I then tried to add a while loop and counters and such but couldn't figure it out. Also, how would I name the new column name?

Thanks if you can help! <3 my output

The functionality you are after is easily achieved using a computed column

alter table stray_kids
add FullName as CONCAT(firstName, ' ', lastName);

Then you can simply

select FullName from stray_kids;

Newer versions of SQL Server also support concat_ws(' ', FirstName, LastName);

If you really wanted to use a function, all you need is

CREATE FUNCTION dbo.fullName(
    @firstName VARCHAR(10),
    @lastName VARCHAR(10))
RETURNS VARCHAR(21)
AS 
BEGIN
  RETURN CONCAT(@firstName, ' ', @lastName);   
END

Note the return type needs to account for the additional space character... 10 chars seems al ittle short though so perhaps use varchar(50)