I found this on redditt http://www.reddit.com/r/polyamory/co...s_parents_ama/
where the daughter is 16, brother is 11 and younger sister is 9 (same structure as my children but ages close not the same). she says she is glad they opened up to her and her siblings rather than hiding the truth from them.
also the young girl herself seems to have a great head on her shoulders and is like my daughter, has not had her own first relationship.
this makes me think dh and i need to at least start the discussion sooner than later.