You tell her as soon as you’re ready for her to dump you — and to recognize you befriended her as a convenience while you waited for her child’s father to come back on the market.
I suggest also waiting until you’re ready to see that, even though she obviously has or had some maturity issues of her own, her dim view of your character will be dead-on accurate — that is, if you proceed with your plan to claim the spoils of not just waiting for this little family to fail, but also actively helping it fail by staying romantically in touch with this [choke] “baby daddy” throughout. (Since when did noncommittal parenthood become cute?)
So when you’re ready to face the reality of being that person, then go for it. If you’re not ready, then please take a hard look at who you’ve become, and how you can turn that around.
Not So Sure Now
I never want to be the messenger of shouldas — useless things, usually — but this one’s glaring: Before you gave this relationship another shot, while you were still discussing it, was the time to draw this line.
It’s not impossible now, it’s just harder because you don’t have the natural, unspoken leverage afforded to you by the open question of whether you’d get back together. Now you have to spell out your terms, including the consequences of not getting what you want, and that does put you uncomfortably close to an ultimatum.
You still need to talk about it — it is a deal-breaker, right? — so take care to frame it not in terms of consequences to him (lobbing an ultimatum), but of consequences to you (articulating a need).
Start by taking responsibility: “I should have recognized how important this was to me before we got back together. I’ve helped leave one of our past problems unresolved.”
Then state the need: “I love you. I also love feeling like I’m at home. I still don’t feel at home at your place — there’s zero imprint from me.”
Then state the significance: “This is starting to affect my feelings for you.”
That’s it. The unspoken question you’re asking here is, “Will you do this because it matters to me?” where an ultimatum would ask, “Will you do this because I’m taking hostage something that matters to you?”
It’s not about the substance of what you’re asking; one half of a couple is always free to ask and the other half is always free to say no and both are free to decide how to handle the relationship based on the outcome of this exchange. What’s relevant is the difference between asking him to give to you, and threatening to take from him. The former stays on your side of the line, the latter crosses it.