All our kids know. Ages 16,16,14,12,11,9,8. We made sure that all the adults were together when talked to the kids so they could see that we were all ok. The teenagers especially were concerned that everyone was allright. The younger kids pretty much wanted to know that they were still loved.
We told them that I love hank, he loves me. I also love darla, she loves me, hank loves her, she loves him, and we all love them. Then answered questions as honestly as it was appropriate to do so.
One word of caution. Once young children know things they are compelled to share. And oh boy do they LOVE to share. We are now out to darlas ex husband, their school, church friends, the barber, grocery clerks, the neighbors, and any random strangers we should cross paths with. So discuss how you plan to handle that.
Me - bi female hinge of a closed V / developing triad
Hank - my husband of 15 years.
Darla - long distance girlfriend of 1 year.
We have seven dependent children between us.