@Radioaktive In the strictest sense, distributing a ROM is illegal, so the RetroPie community cannot distribute an image with the ROMs preloaded. In practice, most ROMs are effectively abandonware and are easy to find online. Many sites have full collections of every ROM for a given system.

However, it is important to not cultivate any perception that RetroPie is used for anything illegal because we don't want this project being shut down (even though emulators and ROMs have been around for ages without being shut down).

You won't find anyone here who will link to ROMs themselves, or a SD image with the ROMs preloaded. However, I wouldn't be shocked if a preloaded image existed somewhere out there. And frankly it isn't that hard to copy the ROMs over.

For whatever city you live in, there is probably a LUG (Linux User Group) where someone locally is probably willing to help set up the Pi for your son for Christmas. For example, here is my local LUG.

http://www.olug.org/

@ferdnand You can find experimental packages (including a few Doom ports) following these instructions:

https://github.com/retropie/retropie-setup/wiki/Updating-RetroPie

Yeah, I'll probably go with the complete reinstall. I think I power the Pi correctly. (powering up by connecting the Pi to the Power Supply and shutting it down directly from the Emulation Station) Thanks for the help anyway! At least it gave me the clue for what might have happened.

EDIT: Yes, the reinstall did its job. I connected to the internet with no problems! I wonder what was the cause... Thanks a lot!

The libretro Lutro seems to be cool too:
http://www.libretro.com/index.php/lutro-easy-retro-game-creation-powered-by-libretro/

MAME:
TMNT - all of them
Metal Slug 2
Karnov - if you like to die a lot
Street Fighter II - pick your version

NES:
MegaMan 1 - 4
Dragon Spirit
Guardian Legend

SNES:
F-Zero
Drakken
StarFox
Super Castlevania
Super Contra

I have exagear desktop on my RPi3b and honestly it's not worth the money currently.

The biggest issue I have with it is the fact that games need to be run in a small window to even make most playable and even then results aren't great. I used it to play Deus Ex, and while i wouldnt consider it unplayable, its certain not great.

There may be better success with other games as I didnt test much aside from Deus Ex (Tried Stardew Valley but didnt have enough RAM on the Pi3)

I believe Floob did a video on Exagear, and tried out some games if you want to see an example.

@caver01
I can't get it to start.
I have tried multiple versions of the game but still cannot make it start.
I run "floob-retropie-attractmode-v03-02-MAY-2016" image with NO updates because when updating I got switched and unusable controller buttons and stick problems.

I will try to do a clean retropie v3.7 with emulationstation only build for raspberry pi 2 and see if everything works. But this will take some days.

Thanx :-)