I would teach Typescript. Being able to write the types down and hover things to see what types they are will definitely help them.
I think C would put them off. I also wouldn't go with Python, in case they want to do things like write games or make websites, which are common tasks you can do with Typescript but not very well with Python.
I don't see why you couldn't have relations in a object model database. Just allow fields that are references to other objects or their keys.