We generally DON'T tell people. Close friends know about our relationship, but we don't advertise nor make mention of things. Family knows A2 as a 'roomate' and 'close friend'. (Well, except for my mother...who's remarkably tolerant of anything that isn't in public and doesn't scare the horses.) If A's family knows or suspects that there's more...they don't ask, and we don't volunteer.
Legally it gets trickier...another reason NOT to say anything. Having a roomate who's not related/married is more common these days than you'd think. (The rotten economy is good for something...) Without actual statements of a relationship, making a judgement solely on basic living arrangements is venturing into speculation, something MOST judges tend to avoid.
As for children...they are far more aware of things than most adults give them credit for. You're not going to be able to keep your relationship secret from your son, so don't try. (Not that I think you would...) Children also tend to be more accepting of things; if everyone's happy, that's all that really matters to them (emotionally).