From reading your post above, you seem sane enough and your plan is a good one. As for NRE, it is certain to happen in just about any new relationship, so you have to factor it in. I personally would advise telling Parter, as soon as possible, that you are at least considering this route with respect to children. It is up to him how involved he wants to be, and he should be able to make an informed decision.
It seems that before pursuing the matter with your Boyfriend, you should find out what Partner's plans would be, so you can tell your Boyfriend about that and he can also make an informed decision. So as previously stated (in your own post as well), the sooner you can let your Partner know of your inclinations, the better. It doesn't sound like it would be you doing any breaking up; he would be doing that of his own accord.
One thing about having kids is it's usually better (healthwise, for the child) to do that when you're younger, which is another reason to get this taken care of soon. You'll have to consider the possibility that Partner and Boyfriend could both split up with you over this, and you could still end up without a child. You'll have to decide whether to take that risk.
Those are just a few of my thoughts. I am in an MFM V, but it is childless and both of the men are snipped. So I am not much of an expert regarding having children.
Love means never having to say, "Put down that meat cleaver!"