Thanks to everyone who apported to this question.
UPDATE:
- I've made it work. So... downloaded Retroarch 1.7.7.apk
- Manually downloaded Gambatte core from buildbot.libretro.com/nightly/android/latest/arm64-v8a
- Added it UNZIPPED to a folder called RetroArch on the tablet's NAND FLASH (the internal memory) placed Super Mario Land rom on a folder inside the created RetroArch folder.
- Launched RetroArch on the Tablet and set the directory route where I placed the Gambatte core UNZIPPED and the ROM route.
- ONLY THEN I WAS FINALLY ABLE TO "PLAY" RETROARCH ON IT
- WEIRDLY ENOUGH ONLY THEN I WAS ABLE TO DOWNLOAD CORES FROM RETROARCH
Before all of this, attempting to download Cores from the Core Updater gave me error: "Failed to load Index" no matter what.
Sidenote: Tablet is so old that I can play NES and GB/GBA roms only. SNES roms loads but they're super slow. So yeah, I gave up on CPS1, CPS2, MAME and NEOGEO.
But considering that I did all of this for a small child, I guess it's okay.
The kid will have lots of fun with Zelda Link's Awakening, Super Mario Land and Pokemon Yellow/Crystal/Emerald and Metroid Fusion!
Thanks for helping guys. I knew I could count with you!
Best Regards!
PS: If there's a Moderator/Staff Member reading this, I want you to choose the best answer for this request. I can't choose because the fix got so criptic and ultimately solved by myself.