If you have teenagers you might want to have some discussions with them about non-traditional relationships to get an idea of their thoughts on the subject. Once you know where they stand it may be easier to gauge how much fallout there might be when you tell them or they find out by chance. (By the way I don't have teenagers so this is purely speculation).
As for the wanting to start new relationships at the same time it's an awful lot of emotion to go through all at once. In theory it sounds like a good idea but if you think about it do you really want to be processing jealousy at the same time as you're supposed to be enjoying your NRE?
Everything will be ok in the end. If it's not ok it's not the end.