No worries… and no question is a dumb question. A rule of thumb when to use a child theme and not to use one:
1. If you plan to make any code changes to any theme file, either now or in the future, such as moving or changing code, then use a child theme
2. If you only need to make changes to styles, such as a font size, then a CSS editing plugin can be used.
To answer your last question….no, you will not have your font size changes disappear on the next update.