Hey! Game is still in works, glad you liked the demo.
I can't suggest any speciefic development kit for GBA development. You might want to install devkitPro - a toolchain which gives you the oportunity to compile for GBA. From that you just go ahead and make your games with notepad++ and tools for graphics and sfx of your choise.
I used this tutorial (which is on Russian), and which have an open source demo what you can use as foundation for your project. And you may also be interested in Tonk tutorial which was used by the author of first one I believe.