Some advice needed on communication.

First some background... My wife and I (together for 10 years) opened our relationship and started swinging earlier this year. After a few dates, we realised it wasn't really what we were looking for. Most of the couples were to eager to get in our pants and we just fell like sitting ducks. After a few months on a website, we also reached out to a woman who was there to try out her bisexual side. She responded that she already found someone and had no interest in dating multiple women, so we left it there. When I contacted her a month later to ask her if she was interested in a party we were involved in, she told me that it didn't work out with the other woman and wanted to meet my wife (but not at the party). They dated a few times, but things went really slow. When she came to our place to bring my wife home, I also met her (she even stayed for dinner and met our kids). For a practical reason (my wife's iphone's battery was empty), we communicated also on Whatsapp. That's when I noticed she made subtle moves in my direction. A week later my wife took her to a private sauna date and the day after, the three of us went to a party nearby. There we noticed the chemistry between me and her. When my wife came back from the toilet, she found us kissing eachother. This was the moment our relationship shifted from girl/girl to a focus on me and her. My wife and her are still evolving more towards eachother, but at a much slower pace (they're still learning about their bisexuality). Soon we noticed it no longer had anything to do with swinging, because of the feelings we started developing and that's how we started realising we had a polyamorous relationship (triad). We date and sleep separate and together.

Some of our friends know about the new form of our relationship and were very positive about it. What bothers us, is we don't know how to tell our kids. My wife and I have three kids from our previous relationships (age 12 hers, 14 mine and 17 mine) and one together of 7. Our girlfriend has kids of 8 and 5. We'ld like to tell them in a way they see it as something positive. We think my oldest son will be the most important factor. His reaction will reflect on the others and we're sure the younger kids will wait for his reaction, to react themselves. We're a bit afraid of how our ex partners will respond if they find out. We wouldn't want them to use it against us.