As one of those mono/poly sigs- in my experience the mono person has to be poly-supportive and agree that it is possible for some people to love more than one person at a time without it being cheating (as opposed to just tolerating a situation in order to not lose a relationship with the poly person). In my hubby's case, his brain just (so far) doesn't leave him the option of loving more than one person. He can be attracted to other people, but thus far I'm the only person he's ever fallen in love with. If that were to ever change, just because I'm poly myself doesn't mean it'd be any easier for me to get used to the new situation. TGIB and I are both poly but we both have times of feeling envy, at least, and wanting more time and not wanting to share, etc. From my pov the success or failure in most relationships, mono or poly, comes down to all parties being willing to put in the time and effort to find whatever answers/solutions will work. But you can't MAKE the other people put in that time and effort- you can only be willing to yourself and just hope that everyone else is also willing to. It's kind of a lot to take on faith, if you think about it. I think admitting that lack of control is hard for a lot of people- like somehow if YOU do all the right things/make all the right decisions (yeah, right, but for the sake of argument) then the situation should automatically work out because of how hard YOU tried and how much effort YOU put in, but ultimately it doesn't work that way.
Sorry, this ended up a little more negative than I intended, but I wish you the best of luck! (I have a 5 year old and a 16 month old, so I totally understand your worry about your kiddo but didn't have any answers to offer since I'm in the middle of it myself!)