IMO you only have 2 choices...

1. Allow them to use it and continue to clean up after them as they seem to use it like a public washroom (which I find so disrespectful towards you...I'm fairly confident NONE of my parents would do this to me!)

2. Put a stop to it now. They are the ones being rude, not you! Of course, emergency washroom trips happen and I'd allow it those times but every day??? Absolutely not!