I don't get whether you have broken up or not too....
I'm going to assume that you are trying out poly to see if it works but from the other side, of having broken up and getting back together to try this out. I think I would just tell the dad of your baby that you will see the other man as you choose and if it is a struggle for him then he doesn't have to be with you any more as a partner and you would be willing to work out child custody. If you are broken up with him then you are free to do as you choose I think.
If you were still together then I would suggest going slowly, making sure that every step of the way the change from two to three is a tolerable in terms of adjustment and the pain and threat it can cause. Dating and partnering up with people one knows is actually a better idea in terms of feelings of being threatened for a male partner attempting to over come the concerns and threats of a partner falling in love with someone else. I know it seems more threatening, but actually knowing the person is really helpful as it takes the threat away. It does the reverse of what is expected.
Not know the person ones partner is with means an image of that person is built up in ones mind and often it is that image that is threatening, rather than the person themselves. Besides, its good policy, I think, to know ones metamours. Don't ask don't tell (DADT) policies in situations like yours usually destroy a relationship, rather than create a stronger one.
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