Nope, it is not absolutely necessary to use a child theme. You can also change the code in Dashboard >> Appearance >> Editor >> Select the footer.php file on the left.
If you do not choose to use a child theme or this editor you will need edit the footer.php in the theme folder each time there is an update.
You also might look into Firebug (for Firefox) and play with that to get what you need before you change any coding. This will give you a better idea of what will happen with the changes before you make a permanent code change.
gk
