Never heard of it. However it is a unity game by the looks of things (
https://slendytubbies.fandom.com/wiki/Slendytubbies_II ) and there is a project to remake it
https://gamejolt.com/games/S2_OS/632591 though it is in progress. It does also mention similar efforts for 1 and 3 being made so those could be a potential source for something -- apparently 2 has some different ideas compared to the other games in the franchise though according to that page.
The DS does not have Unity ported to it so you will have to make your own 3d engine to handle whatever features are needed.
https://devkitpro.org/ is the coding method of choice for most would be DS developers, that will be the C programming language though I imagine you will also want to know some assembly to get it running any kind of speedy.
Alternatively I don't know whether the various dementium games or moon or something might be a suitable basis for a ROM hack total conversion to play something like this game. Could be an option and spares you having to make a 3d engine, or adapt it for one of the existing open source ones.
Whatever you end up doing this is a fairly major project, and if you are asking for source code to a presumably closed source game then you are going to be facing an uphill struggle.