It's one of those things where, if it's that important to you, you have to say it upfront. When people say things about kids and marriage and stuff early on, it's not because they're saying "let's have kids" or "let's get married" it's because they're trying to give you an idea of where they would ideally like to end up one day. Her saying that she wants kids with your husband isn't a "get me pregnant NOW" it's a question about the future - will I EVER be allowed to have kids with him is what she's asking.
If it's a no, then you've got to tell her because that's a pretty big deal. She should be allowed to make her decision to enter a relationship with you based on all the factors.
Whether or not it is common or uncommon in poly relationships is completely irrelevent, because you've each got to do what's right for yourselves, ESPECIALLY when it comes to future kids.
This is sometimes the problem with primary/secondary classifications, the primary couple assumes that the needs of the primary couple are more important than the needs of the secondary. What should really be the case is that the needs of everyone are met, and primary/secondary refers more to the live in situation, how much time you all spend with each other etc. It doesn't mean that someone doesn't get the right to have the things they want - if she wants to have kids with your husband some day (or the option to at least) and you're not willing to provide that, then it's not a case of the needs of the primary outweight the needs of the secondary, it's a case of three people not being compatible together.