contact your local countys health and human services dept. there is usually training involved, and they will help you figure out what to write up in a contract. also, this will give the parents the ability to apply for county assistence if they need it; that will pay their child care, or a portion of it. you dont HAVE to accept assistance by the way, but its an option that i liked.
otherwise, a contract should have the important information: hours and days you are open, your rate, holidays/whether you have paid holidays, man, its been so long, i cant remember what all i had in there. (i quit this year).
you should/could also have a "policy" which would include simple things like whether its ok to send food, or drink, your activities/schedule for the day, whether you are involved with a food program (for licensed only)... etc.
but anyway, it doesnt sound like you've taken any training or anything or you wouldnt ask this question i dont think. so contact your county's health and human services and they will get you in touch with the right people.
if you just want to keep babysitting this one kid, you can write out something breif for her, but im going to warn you right now, it can get sticky when you are friends with the parents. thats one thing i was always warned about; once you get to be friends, then it feels wierd to ask for money, or remind them to pay, etc etc etc. so im just warning you, if you and your friend cant talk honestly and frankly about this its going to make things a bit complicated.
you can just write up your concerns for her and meet with her sometime. go through it with her, and talk to her about things. get her to sign it, and really, thats supposedly a "legally binding contract" as long as she's signed it. i think.
anyway, its a lot of complicated situations and stuff. i hope you got some better info from someone else! LOL