it depends, i had a really quick look(more like click on a link or 2) and they look alright(seems like they did some of the stuff for u), as long as u are like what ZAFDeltaForce has mentioned, fluent in programming language such as C/C++/etc, u should be fine.
You will need to devote alot of time reading up on algorithms for pathfinding(if your game requires ai to look for u in a room or something like that)/AI (for your games of cos, unless u are making a game devoid of AI, like a painting tool or something like that).
Asking us on whether or not making a ds game is hard or not, is a tad subjective, your best bet would be to actually go to ds programming forums and ask them yourself.
and to sum up what i just wrote, what he said.
O ya, IMPORTANT: you HAVE TO make sure that u are not violating any copyright when u add songs/sprites to your work. If u are just making a game for yourself, u don really have to care, u can just put whatever u want, BUT!!! if u wish to share it on the internet, MAKE SURE!!! that your stuff are free to use and have no copyright string attached, since i doubt that u will want to meet lawyers on a regular basis.