My ex moved into an emergency shithole flat without viewing and only found out it was a macerator toilet afterwards. Odd set up, was an upstairs bathroom but in a house where it had been split vertically into two properties. He was in the front half with no back yard etc, so his bathroom was like an ensuite, no soil pipe, it connected to the neighbours soil pipe. It had no window, and a broken extractor fan. He had no idea he couldn't use toilet wipes with a macerator, or certain cleaning products, until i told him.
I HATED visiting him. As someone with toilet related severe OCD, i use excessive paper even for wees, plus i have IBS and typically need wet wipes so was taking immodium before every visit as the distance meant an overnight stay and i struggle even weeing in public toilets to go on the journey home. I tried that wype gel that makes regular loo roll into wet wipes, it did not work well at all. There were a handful of instances in the 18 months he was there that i just had to go, and the anxiety of it was awful. Including the fear of a power cut meaning no flushing loo!
I don't think the majority would intentionally use a downstairs toilet for a crap (minus my late grandma once at a family houseparty, it was, memorable) because it's obvious it will cause a stink that lingers, most would surely be more comfortable going upstairs where nobody will hear or smell their bowels, so i doubt a sign is neccessary.
You do need to accept though that it is going to happen sometimes. Your daughter should get a free pass really, plus it could be unavoidable if the upstairs bathroom is in use and someone really needs to go.