You really need to think long term here. You have absolutely everything that most people want and dream of. You have created a happy family with a long term partner who you still get along with and still have intimacy with. This is not easy to come by, and not something you will likely come across again. Even if you did come across another long term partner you were happy with, they wouldn't be the father of your children. That is a unique relationship you have with your current partner and one worth fighting for.
It might be purely because^^ you have everything that you aren't appreciating it currently which is a shame. You may feels it's easy come easy go but truly nothing can replace the relationship you have with the father of your children, and I would be saying that even if you were on the rocks. They are the only person who knows and loves the children the way you do, and raising them together will always be different to raising them alongside a new partner who maybe has their own kids, their own ex and baggage, and doesn't love them the way you do. On top of the fact that you are getting along great with regular sex and no major issues! You would be crazy to throw this away for some casual sex.
On top of that let's think morally. Your partner has done absolutely nothing wrong. They have dedicated a huge chunk of their life to you. Been faithful to you. Raised children with you. Loved you and continue to love you. They trust you. How could you even consider betraying them? To cheat on someone in this way to truly to destroy them and to destroy their future relationships also. You will leave a wound that will last them a lifetime. It will also impact your children. Their perception of you, your partner, their idea of healthy relationships, their happiness and their shot at an intact family unit. Your co parenting relationship would no doubt be a mess if built after such a betrayal.
Reality is what you have done so far is already cheating and a huge betrayal. For me the relationship would be already over if I found out my partner had done what you have done so far. The best you can do it stop it here and block their number.
It's a shame because you will have to live with this secret and betrayal now knowing that if your partner knew the truth about you and who you are and what you have done they might not even want to be with you. That's bad enough to swallow and I would leave it there, take this to the grave and learn to appreciate what you have and cherish it fiercely. See this is a near miss, nearly losing everything you have.
However, if you can read all that and still be tempted at all then I beg you to end your relationship now, before you completely destroy an innocent person. Then you can do what you like. See how quickly this little fling and being single gets boring and lonely.