Welcome to the poly boards, but I am sorry you find yourself in a difficult situation.
I think that if you read here you will find that many people have found the "Relationship Broken, Add More People" model doesn't tend to work out too well. I agree with those here that will advise you to seek counseling with your husband and fix what is wrong with your marriage (if it is fixable) before drawing someone else into the mix and complicating matters further.
It may be that you and your husband don't work well as husband-and-wife but do work well as parents (this would be the case with my own parents - great parents, mediocre spouses - they "stayed together for the kids" - not sure they wouldn't have been happier in the long run if they had separated). In which case it may be better to separate before you expose your kids to too much drama. Just because you love someone doesn't necessarily mean that it is good for the two of you to be married. What about a scenario where you separate and co-parent the kids and still love each other while exploring other relationships in addition to your own?
Just a thought based on the (very) limited info in your first post.
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe