First, younger children are a matter unto themselves. I find, as a rule, that children, especially before pre-pubescence, will act out what is unconscious in the parents. Hidden antagonisms or desires, as well as ommissions are often processed behaviorally by children like they were barometers. I have stopped being astounded by this phenomenon. Yet too, their processing of feelings takes a long time (really, just like us). This should be recognized, in particular, if a person comes into their life, is given the child's affection and trust, and then through circumstance, they disappear from their lives. This is something I am very sensitive to in developing relationships. Building up expectations of involvement in a child's life and then having that person suddenly go poof is going to effect their attitude towards Polyamory. We are therefore more than a little cautious about having someone jump into a role that can't be followed through.
Poly mantra #1:
Go Slowly and Communicate, Communicate, Communicate. Having some idea of how the content of our communication matters underlies this. And too, skill at expressing and negotiating expectations, emotional intelligence, and that little thing we call, insight and self-awareness are very important to any relationship, eh? It would be impossible in a discussion to even outline the knowledge required to properly answer all the general questions. All life is full of critical episodes, conflicts, life tasks, and interpersonal modes that make life interesting, often as not, very exacting, and thus full of creative tension. Communication with children is about mutual repect, the kind that is culturally enriching andexpresses how important it is to you what the child thinks and feels. That sort of respect gets reciprocated. Of course we all do not have the same tendencies and inclinations. Being aware of differences, I'm reminded that there aren't any "fits all sizes" when it comes to these things.
To my mind what goes on behind locked doors that adults are doing is private. Everyone up to and including one's primary(s), relatives, kids, etc. being into everyone's business can be a bit too incestuous, to my taste. Honesty is important but where all become so entangled as to not allow each their own need for some boundaries and a sense of inner privacy seems, well, suffocating and unhealthy...in my considered opinion. Still, children do absorb a lot and sometimes hold some strange ideas close to their chest, so promoting dialogue may help to alleviate unfounded and ungrounded ideas that they might have.
Younger children are pretty flexible and accomodating on the whole, though there's always individual exceptions. But pre-teens and teens are another animal. Sometimes they'll surprise you what they can accomodate. There's the issue of this age group being excrutiatingly sensitive to group pressures and collective values. I remember myself feeling angst at anything that identified myself with my own "weird parents." And no, you don't even want to go there.
The relationship of adolescents with parents is sometimes fraught with intensity due to the task of developing a separate identity... not always easy for them or parents under any circumstances. They are developing inner controls and beginning to figure out that parents and other authority figures are fallible and human. One walks an edge between being firm and nudging a child on, and being overbearing and pushy oppression, between promoting self-acceptance and being yourself and over-indulgently not letting them test themselves againt life's raw realities, thus becoming lazy. But if they are burning effigies of you in the back yard it might be a good idea to get some idea of what's up with them...
Including children in some level of adult realities is worthwhile. Allowing them to verbalize and process their feelings is important. My experience is that especially in adolescence they need (and are happier with) defined limits but also need to "try on" independence. So I would not give them a controlling, quorum vote on some matters of the heart. That's really a situation of each family's dynamics, and we all have to play things by ear, instinct, and intuition...case by individual case I suspect that many who have come as far as "owning"*** Polyamory already has less inclination for being hidden and secretive. So the kids are probably, in some way, acclimatized and some may be far ahead of us on accomodating things. My "philosophy" if you could even call it that about these things is that if we expect better then kids will live up to that. This may seem "wishful thinking" and to fly in the face of a sociological "problem-solving" and the "adjustment" mindset and approach. I tend towards nurturing, reassuring, and tenderness as central, but not to render a child inactive and dependent. This is effected by what sort of unified front that parents have developed. Children will test, manipulate, and carry attitudes that reflect their parent's relationship to each other and their parent's unconscious attitudes towards eros. I would like my own children to listen to their own free and natural feelings. I don't feel that love arises from obligation and commitment, nor as a response to institutionally imposed authority. Assigned social structures need to come from within the heart of the individuals involved. Yet too, unrealistic "love affairs" no matter how blissfully they begin may end badly... So discrimination and judgement are important guides. Children are a good reason to proceed with a mature and balanced outlook.
As far as what is shared in school and with others, like family and friends. It begins with how comfortable we are with our own, real choices and if we deliberately approach our own life situations as something meaningful and acknowledged by ourselves with esteem. That is the only satisfactory answer I can make. Teaching discretion is really a personal matter for each person to make. What we reveal or keep to ourselves is about discriminating and making judgements. I feel that showing some open expressions of affection, with some discretion, and including children on discussing plans and being straight about who will be guesting or staying is healthy. These are my principles; if you don't like them I have others.
A suggestion, don't start a big "family discussion" on this if you haven't usually convened such things before. My 2 cents worth of advice is to feel out other matters that are of concern to all first. It gives time to feel out how such discussions will go, and develop everyone's skill in "talking specifics."
***"Owning" means something like but not quite "outing." More like assuming upon oneself the responsibility, values, and self-described "ownership" of living as a "polyamorous" person. It's an idiosyncratic word that I use on occasion to describe the living-terms of standing up for what one uniquly is... as in I "own" that I am something that is considered a counterculture.