Any idea on when?

That Kinderland coach is awesome! :-)

Its not clear from the image of the Kinderland car, is its interior modified to have the play area in place of the normal seats?

We will have to see, but I am pretty sure it does not. The Eurotrain coaches are usually repaints of existing models with no detail changes.

I think this will be a job for the kit basher. I thought about covering the seats in the play area with a sheet of black paper. After all it is difficult to see interior details inside the model, but you will see the seats. Black paper will indicate that something else than seats is there. Which is good enough for me. Additionally, you could also put a playground slide in the idle space in the aisle between the paper covered seats. It might just work. I am going to try it.

That is a very nice kitbash. Very detailed. Respect. Obviously, mine will not be nearly as advanced.