May I also add, that anything DSi-related would very easily fit into Nintendo DS, because they are technically in the same console era. Just that the Nintendo DSi has a slightly improved SoC (System on Chip), to use a few more instructions than what the SoC in the Nintendo DS and Nintendo DS Lite used.
Imo, there is no need to create a sub-section for Nintendo DSi. I'd say labels would make it more easier for anyone to find anything DSi-related. Now there should be some function that can be enabled for the Nintendo DS section in where labels can be added next to the thread title, so that users can hide all threads apart from threads with the "Nintendo DSi" label next to it. I've forgotten what that forum term is for labels next to the thread title. Was it header? Label? Highlight? I don't remember very well.
Anyways, with that said. If there's not enough activity to create separate sections for those consoles, then the only solution is to keep it within one section.
How about, the forum section could be renamed to "Nintendo DS/DSi Hacking & Homebrew"? Wouldn't that be a better idea? That way it won't confuse everyone into thinking that it's only for original Nintendo DS. There we go, problem solved!