Re: Telling the kids.
CP, you can get into this indirectly, and probably should rather than wait until you're ready to announce an engagement or a marriage. In most cases you can get into it through bible study, or a "did you know?" type of discussion of how having more than one wife used to be pretty common and is okay with God. Let them get used to that idea before they have to deal with the practical application.
I think FH also mentioned just having your prospective second around the house a lot will familiarize the children with her and hopefully lead to a situation where they really enjoy having her around and would welcome the opportunity to have her around more.
And I just mentioned this in another thread, but given your location on the ground you might consider coming out and visiting my family some weekend, or whenever you can get away. Might even kill two birds with one stone, as in using "we're going to go visit some new friends of ours, and they're a little different" as a reason to get into a discussion about how hardly anybody in our culture lives like this anymore but that doesn't mean it's wrong in God's eyes.