I'm sure this applies to men dating bad dads also, but I have watched many a female friend, acquaintance, and/or co-worker go through this. In a (vain, I'm sure) attempt to save somebody somewhere some heartbreak, here we go. If he's a crappy dad to the kids he already has, he will be crappy dad to yours. Period. The only exception I can think of to this rule, is if his kids from other relationships have been kidnapped, are in the witness protection program, or some similar reason makes it actually impossible for him to participate in their lives.
So here we go. You begin dating this guy. (And actually I'm sure this advice applies to women who are bad moms to, but again I am defaulting to framing this how I have seen it played out.) You discover that he has child(ren). Assuming this is not a deal breaker for you (hey - for some people it is), you inquire more. You discover that the children (I'm going with the plural here) live with their mother (or some other relative) and he doesn't see them.
This does mean that you won't have to worry about the kids interfering in your lives. This does not mean that he will have plenty of time to tend to kids that you have or will have. No, it means that children are not important to him. That they are your children and not those of the horrible women who trapped him and shriveled his soul will not change this. If he wanted to be a dad, he would be. If having a child was all it took to change his outlook on this, that would have happened already.
I'm not saying that people can't or don't change. Because they do. But, like anything else, when you are presented with a behavioral pattern in your partner, it's silly to expect that it will change, absent proof. Such as his taking an interest in his existing children.
Sample scenarios (the names and situations have been adjusted so as to protect those involved).
Scenario A: Jessie and Justin.
Justin has two children - a boy and girl. They are four and six. They live with their mother Jenny. Jenny and Justin have worked out a deal (without legal action) wherein Justin sends a specified amount of money on a monthly basis. Justin lives across the country from his children and has not seem them since he left Jenny. So Justin and Jessie have two kids of their own. Except that, Justin feels the kids are more Jessie's responsibility. He considers his participation in their care to be babysitting (not parenting) and refuses to sit for more than one at a time. Even if Jessie just needs to run across the hall. And then the relationship started to fall apart (for other reasons). And Jessie had to file for child support since Justin didn't want to continue paying for the kids he wasn't living with anymore. He visited a few times, and then disappeared.
Scenario B: Sandra and Steve
Steve also has two children. They also live with their mother on the other side of the country. Steve does not visit them. Sandra and Steve have a baby. Then, when they are forced to find a new place to live, Steve does not come with Sandra and baby. Sandra doesn't hear from Steve for a while and begins dating other people. Then Steve shows up again and they resume their relationship - which leads to another baby. Except that as she approaches her due date, Steve becomes distant. Sandra files for child support with the courts. Steve reappears and moves back in. (As you may know, you cannot request child support from someone you are living with.) And then he leaves again.
Scenario C: Nick and Nessa
Nick has two children. He shares custody with their mother Nora. This one has a happy ending.