The going rate for under the table babysitting (where a kid just babysits and gets paid cash, no monthly paychecks or paperwork) is 5 bucks an hour for up to 3 kids. IF there is a baby, then the rate jumps 2 bucks per baby. So say there is 1 baby and 2 children, then it would be 7 bucks. Babies are children under 3.
That is just around here though. 7 years ago I got paid 5 bucks an hour to watch 3 kids every day during the summer, made a killing off it too. But for some reason, the rate hasnt gone up in years. My mother in law just got done with a month of babysitting a 9 month old and she got paid 2 bucks an hour....which is horrible.
EDIT:: Really it is something you need to sit down and talk with the parents about. They may be on a lower income bracket and cant afford much, and it all depends on how many kids there are, and you said you have another person helping you, so your cut may be cheaper. And your best bet (if the parents are ok with it) is to get paid seperately. That way the parents dont give one of you the money to split amongst yourselves and one decides they did more work than the other and should get more. So yeah, sit down with them and talk to them about it. They wont freak out cause you want to talk about money, they understand that babysitting is not free and are probably expecting this talk.