I have kids and have faced this dilemma myself.
How I handled it was I was open and honest. I was frank about my boyfriend and I relationship without being sexual.
I explained that Murf was my boyfriend and I loved him and he will be apart of our family. And that I also still loved my husband Butch that nothing was changing between he and I.
My 10 yo son asked how I could have a boyfriend and a husband at the same time. I asked him if he loved both his brothers. He said yes. I then asked him if he loved his brothers the same he said no. I then asked does that mean you love them less he said no. I asked the same question about our dogs. Then asked him to explain the concept back to me. He did... and I told him if he ever had any questions to come talk to me or my husband.
My advice is be honest... Keep sex out of it... be open and allow for questions.
Late 30's staight female
Married in the eyes of the government to Butch since 2001...
Murf my second husband has been with me since May of 2012.
In a V relationship with an average 60/40 split of time. Only due to Murf's and Butch's crappy work schedules.