Yes, I would agree with getting to him first, before you move. Perhaps you will get a sense of what might happen in the future by doing that. If it doesn't look favorable then I think I would probably hold off from moving. Where my child is concerned there is no option that his happiness be put before mine.
That or just do your damnedest to keep it all off facebook and out of your conversations to anyone suspicious. Talk to your daughter, if she is of an age to, and keep quiet until you have settled in for a year or so and then see where things are at.
You can make another FB account no? With a new email? That way you can have a Poly friendly FB account and a non-friendly account to. Be careful who you "friend" though.... Another option? Go into the "settings" at the top of your FB page and privatize the hell out of everything. Block people from seeing things.... be careful though as this doesn't always work on certain occasions....
Oh the thought of losing my child just kills me and fills me with absolute terror. I feel for you! hope it all works out.
Just a note of confidence.... I am like you and unable to keep my own secrets. I have kept this one for 9 months now from my family, once you get into it, it really becomes easier to just pretend you are all good friends that hang out together... or in your case, just live together. Really, it is all financially more practical in this day and age anyway riiiiight?