The best emulator of nds is no$gba.
:thumbup: the most advanced emulator of nds
:doh: the debugger is not free (15$).
:doh: it lacks source level debugging (you can buy it for 1750$ :shock.

Another way to debug is to use the new GDB stub for Desmume emulator.
:thumbup: free and open source
:thumbup: it works with gdb/insight, so source level is allowed
:doh: less advanced than no$gba
:doh: at this time it's very unstable

However no plans for a demo yet