Good suggestions so far! That's what I was thinking about "dumbing it down" for them. Basically if they are going to act like children, I guess that I'll have to treat them that way.
To get into specifics, they'll do things like:
-not tell us when they are coming, how long they are staying, when they plan on leaving - they just show up and expect us to accommodate. Getting this information is like pulling teeth.
-rummage through our pantry/fridge, looking for and taking food without asking - when there is other food available to them, its just not what they want. MIL often takes it upon herself to prepare meals when she's there, which I feel is presumptuous, but of course I often don't have anything planned, because I never know when they are coming and if they will want to eat (see point 1) There are also some picky eaters in the bunch, so if they don't like the food that has been prepared for everyone, they will just make themselves something else
-turn on the TV really loud when we are visiting.. to programs that neither husband or I want to watch. If it was one show they really just don't want to miss, I understand, but they'll watch reruns for hours instead of spending time with DH. I can tell it upsets my husband that his family would rather watch TV than spend quality time together as a family, especially because they don't see each other very often. (I'll hide the remote this time - at least make it a little harder!!)
-interrupt me or my husband (or each other!) when speaking - they have a habit of just yelling over each other until it just gets louder and louder and I just give up on speaking all together because I don't want to participate in a screaming match
- criticize my decisions, life choices, home, preferences, etc - all under the guise of "being helpful". (I can let this one slide.. I think that just comes with the joys of in-laws!

)
- funny you say about the medicine cabinet - yes!! they totally do that! last time they stole my shampoo out of the shower

Frankly I don't care if they need to use it, but a) ask b) put it back! Not fun to be soaking wet in the shower trying to get ready for work and realizing you have no shampoo. lol
- when they stay, seems things to get damaged frequently. I often don't see until after they are gone. There's never an acknowledgement or apology. It's not that these are fancy or expensive items, but they are ours and I don't think I should have to replace/fix multiple items after every visit.
-expecting us to provide toiletries and specific food items (even things we simply don't use and never have) - they seem surprised/offended when we don't have something, and I have been asked to "run out and pick up xyz because you are out"... of course we are out! We have no use for it! That time I did go get some, because I was at the store anyways, but it annoyed me that they feel they are entitled to it and because they are staying with us I need to provide all food and personal items free of charge. I'd say we are treated like a hotel, but hotels get paid! ha ha