Depending on the kind of games you want to write to a microSD for your son,
consider the following:
Nintendo DS via DS_GAME: Displays files from the directory containing the last selected DS game, so you can sort of restrict to a single directory yourself first. 9 files on a page, after which he will have to scroll.
Game Boy (and Color) via Lameboy or GameYob: Displays files from /LAMEBOY on the card by default, configurable in an .ini generated after exiting the emulator for the first time. ~15 files on a page, after which he will have to scroll.
Game By Advance via TempGBA: Displays files from /TEMPGBA/GAMES on the card when New Game is touched, non-configurable. 7 files on a page, after which he will have to scroll.
Super Nintendo via CATSFC: Displays files from /CATSFC/GAMEPAK on the card when New Game is touched, non-configurable. 7 files on a page, after which he will have to scroll.
(nesDS: see above)
If you have more games than these page sizes, your son will have to know how to scroll. Most of the time you can just keep pressing the Down direction though. One entry in most file choosers is the parent directory, "..", and I removed this entry from my counts above. DS_GAME, nesDS, TempGBA and CATSFC also support touching the desired file, but not Lameboy or GameYob.
Good luck, and have retro fun!