I would make a special mention that you'd prefer the kids not use craft supplies in their reach, show where the okay ones are as well as smocks, etc. I'd also ask to make sure any clothes dirty with art stuff just get put on the washing machine and/or mentioned in case they need stain treatment. I think with a little talk about that stuff, this is unlikely to get repeated by your sitter. If your dd mentioned they were her paints that does seem like something that becomes okay to do, etc. as pp's mentioned. If you don't want the nightly baths, I'd comment that you've noticed the frequent bathing is bothering your kids skin, please not tonight on any specific night.
I'll add that, as a teen babysitter, I wouldn't have gotten out paints or done baths unless it had specifically been mentioned to me by the parents - but I was used to asking or not doing things that way at my own home, and if people are used to regular kid baths or not having to ask about supplies they'll do things differently.
As an artist - I feel badly for you about the paints . It just stinks to have something like that used improperly, with the expense, etc. . .
I have materials like that I use with our kids, but it's definitely kept away/out of sight. I probably over-lecture dd about caring for her brushes and stuff like that in the first place - but I have cheaper stuff that I leave out that I'd I might mention to a sitter or something, but even dd (who is 4) knows that I need to get asked about the art stuff and that it requires a certain set-up - smock, art mat for the surface, etc.. Maybe going over that with your kids (with the purpose that they can do more of it on their own anyway, even with you around) might help too? - they could help self-enforce stuff like having their smocks, taking care of stains on their clothes.