Where can we do laundry in Warsaw? Or where can we take it other then the hotel?
Perhaps you can send an email to your hotel and ask if there is a laundromat nearby. Not that you asked for advice about detergent, but I collect detergent samples before my trips and carry them with me so I can do laundry without having to worry about purchasing it there.
I'm sure most established hostels/hostels have laundry machine. Warsaw is a big city, you should be fine.