Welcome to our forum.
It sounds like your biggest challenge is your new boyfriend's fears/insecurities about the situation, though I know you say the kids is a big concern. The oldest is nearing that age of moving out and getting his own place, so in theory he should be "live and let live" about a lovestyle you and your husband have both agreed to and are happy with. I know practice doesn't always measure up to theory ...
I can relate to you about being "in the closet" ... I've been there for over six years. It's tough at first when you wish you could shout it from the rooftops, but you adapt to it after awhile, I guess. It's a shame that polyamory isn't accepted in our "modern day" world, but perhaps someday it will be. If you come out, you do; if you don't, you don't. It's totally up to you, and always a tough judgment call.
Just take things slow, and I'm sure everyone will acclimatize to the unconventional situation over time. Lots of good communication is always important.
Glad you could be with us on Polyamory.com.