We were on soak away septic and when I met my husband, he had a whole little written blurb in the bathroom about proper use of the septic for ALL visitors.
The other things- I would probably sit them down and have a discussion with them about the various issues....or keep reminding them each time it happens.
Re the FIL leaving your daughter outside alone, I think I would probably just make sure I didn't leave him supervising my kids if he doesn't understand that he shouldn't do that.