Signs for when we have visitors - mainly the IL's

So I have the IL's staying at the moment and everytime they visit we have the same issues every time they are here - even though DH has told them every visit what they need to be aware of, especially now with 3 toddlers running around the place.

So I'm thinking of making signs to put up in certain areas as reminders....

My biggest gripes -

Leaving the toilet seat up - My sign would say "There are kids around so for safety put the seat down" (our toilet is in the bathroom and DD can open the safety gate we have across the bathroom door to use the potty and sometimes she leaves it open and the twins follow her in)

Not flushing the toilet (added to the leaving the seat up, equals eeewww) - plus we are on septic and if you keep loading the toilet up with toilet paper everytime you go but not flush the pipes get clogged up and we get septic flowback.

Turning the kitchen taps on when someone is in the shower - a couple of times they have done this when DD was in the shower with DH and could have had some serious burns.

And my latest is DD loves to play outside and we now have a trampoline and wading pool up for her to play in when supervised, so DH has asked FIL to watch DD if he has to come inside for a while and within 2 minutes FIL follows him into the house so DD is now unsupervised around water and if she is on the trampoline and wants to get off is now about a metre off the ground trying to climb down.

So I would love to put little reminder cards up for them as to what they need to be aware of, do you think nicely worded reminders would be too rude as just telling them doesn't seem to sink in...

This is tricky. I guess it depends on your relationship with them and whether you think they would take the notes in the spirit they are intended, or if they would be mortally offended by it?
Perhaps you could make it more like these are notes you've put up to remind yourselves (as well as any visitors) so that they don't feel overly targeted by it?
Edit to add could you use one of those toilet seat locks? And a little sign saying please flush after every use?

For the safety reasons you have outlined above I would certainly put them up around the bathroom and taps. If they ask say they are for friends who sometimes visit.

The outside thing is probably harder. I don't think you could leave the kids out there with them if they can't remember that kids at that age must always be supervised around water. Makes it harder I know.

I would feel really offended if my family started putting up reminder cards for me, sorry, but I wouldn't.

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.

What NicNac said. It's your house, your rules and many of these are safety issues. I think a sign for all visitors about bathroom and tap etiquette is fine. I wouldnt leave your kids with your FIL if he can't be trusted to supervise their safety.