Well... the DS light plays both GBA and DS games but I'm guessing you need it for 3DS or 2DS.
You could buy a 3DS R4 card, download an Emerald GBA rom + all the software required to let you play GBA games on it since it's not designed to play those games. You should look into this more though and not take my word to it.
You could also play GBA games on your smart phone by downloading MyBoy - Free! "for GBA games", MyOldBoy - Free! for Gameboy Colour Games.