Our friends were honest with their kid (admirable) but then got outed by their kid (caused a lot of flak). They did not blame the child or anything (also admirable) but now there's a lot of relationship damage to be dealt with. I think they went with the "let's be honest and deal with it when we get there if we get there" approach. Fair 'nuff -- it's their fam.
To me it's easier to just be closed in the parenting time. I don't think it's a horrible thing to be poly in the parenting time, it just is not a space *I* want to deal in.
If I were going to be open in the family/parenting decades of the 30s and 40s I would be honest all the way across. From the start -- I don't like the "deal with it when we get there" because I don't want that kind of random emotional storm. I rather prepare, hit it, and get through it. I do not like it sneaking up on me.
Easier would be to be out to family and friends BEFORE having the kids though.
Young kids are blank slates -- they are very accepting. You being out to the kid is not going to be a prob.
But they have kid brains and internalize things sometimes and do not understand or process like an adult.
I'd hate to put the kid in the rough spot of feeling like they have to choose between allegiance to the parents and allegiance to say... the grandparents. Because the grandparents are not down with Poly and can't just leave it alone and agree to disagree. That's emotional brouhaha that a young kid could be spared.
That said, sooner or later it comes to their awareness that there are alternative family structures. My kid covered that already in her sex ed class -- adoptive families, lesbigay familes, etc. To her poly just seemed to be part of the mix of possibilities. I like "Our Whole Lives"
for the honesty in sex and relationships, although I have to do some home bonus info to round out places where I think it does not stretch enough. But of all the things that is out there, that's the best I've found.
I dunno if that helps.