I agree with daletom. You guys are getting married. By doing so you accept her, her child, and any financial implications. When you are married, you don't split things. You do it together. My suggestion is to get a joint account where you can both put money in for bills, etc. Then you can have your seperate account for whatever. That's the best way to do it.
It kinda disturbs me that people are saying the kid is hers so she should cover his expenses. He's a human child, not a puppy. When you marry her, you're accepting both of them into your family. Expecting her to pay for 'her' kid is ridiculous. It's no longer just her kid, he's your kid too.
As daletom says, the spliting rent thing sounds more like a contract you make with your roommate. It's a bit childish. I'm really not trying to offend you or get on your ass, I'm just letting you know what I think of the situation. Maybe you should talk to her about it and see what she thinks. And PLEASE don't say anything like, "So are you going to pay for your kid?" because I can guarentee that she will be offended by that!
Hope this helped out, and good luck! I assume you haven't proposed yet, so when you do, hope she accepts!
Go to Heaven for the climate, Hell for the company.