I want to convert a 1st floor room into a bathroom, retaining the original wooden floorboards. There will be no shower, just a bath in the room. What needs to be taken into consideration with regards to waterproofing the floor to prevent splashes, spillages and kiddie bath time from ruining the ceiling below?
You could sand down the floors, and refinish them with oil based stain and polyurethane. That should be enough protection to keep the floors from rotting. You could do what they did in the old days and put towels down around the tub and wipe up spills immediately. Standing water will leak through the cracks between the floorboards.
You could also good clear polyurethane caulk meant for bathrooms and fill the gaps between the boards around the wet areas, or on the entire floor. This should prevent water from seeping to the room below.