Ask her what she would do if one of the kids got hurt or if there were a fire.
There's minor fell and scraped their knee kind of hurt and then there's the more rare and more serious hit head hard, or needs stitches kind of hurt.
Write down your address so if she needs to call 911, she can tell them where she is. Also clue her in if the kids have any allergies.
Make sure she knows where your fire extingiushers are and how to reach you in an emergency. Also if you have neighbors nearby that could help if needed.
Also, lay down the ground rules about things like phone and internet use, and whether she can have friends over.
You can throw that in matter-of-factly with everything else. Something along the lines of "it's fine if you need to use the phone, but please limit calls to 5 mins or less, and we'd rather the children not go online when we're not home, and we prefer no one else come to the house while we're not here."
Of course, if you don't mind the sitter or your kids surfing the net, say so. My kids are allowed to go to certain sites and the sitter knows she's to monitor them while they're online.
Finally, there's the food issue. Teens like to eat. Most won't help themselves to your pantry unless you okay it first, but others will eat anything not tied down.
I always tell mine to help herself (so she knows it's okay - and I would feel bad about her feeding the kids and not having anything for herself) but if there's something special, I let her know if it's off limits or not.
ex: the cake is for my sister's birthday tomorrow. Don't let the kids cut it tonight
or - I baked a cake this afternoon. Feel free to have some.
Also, let her know what sorts of things you allow the kids to eat or not eat, and where they are.
ex: there's nachos in this cabinet and salsa in the fridge. There are grapes and oranges in the bowl on the counter, and Little Johnny likes apple slices with the peel removed. If he wants a sandwich, he likes it cut in half, but so that it makes two triangles, not two rectangles (or whatever preferences your child has that others wouldn't guess easily).
Finally, if she'll be putting them to bed, make sure she knows their bedtime routine, where their pj's and toothbrushes are, and how much help they'll need getting ready for bed.