Things like disagreements over children could pose an issue. You said that he is done having children, but you believe the door should stay open because of this third person. You sound like you want to seek someone who might want to have children, but it is his biological right to not provide his half of the DNA needed for any supposed children. This is something that needs to be talked about now.
I know that you are just figuring out things, and the best way to do that is to ask questions. I guarantee that we will all answer honestly.
I advise you both to talk, talk, and talk some more. When you open relationship, there is no turning back. It will never go back to the way it was even if you decide to close it and continue being monogamous. You both need to be on the same page. I would encourage him to join on here as well and ask questions, too. Keep reading the various threads. The Lifestyle and Blogs section is an excellent place to really get an idea of what you might be getting into.
Why do you feel selfish? I am not going to lie to you. It is hard work, and anybody trying it has their work cut out. Even the seasoned people still have their work cut out. No two relationships or experiences will ever be the same.
To give you an idea of what a triad would encompass--relationships wise--is as follows:
you + K (alone time/romantic time/bonding/intimacy)
you + her (" ")
K + her (" ")
K + you + her (You all have to communicate.)
That covers the adults. Those relationships all need time, nurturing, effort, and energy. You cannot combine them into one big relationship because no relationships grow at the same rate. Your core relationship with K is just one piece to the poly puzzle.
You mentioned that you have children. I do not know how old they are, but I am going to assume they are under 18. If not, then this would not apply totally.
you + your children (mum/children time)
K + your children (father/children time)
you + K + your children (family time)
you + K + her + the children (poly family time--if that is what is desired by all)
the children + her (If you and K decide that you want her to be involved in their lives, and if she would want to be involved as an aunt or something type of role. Your children may/may not want to be around her. That is their right.)
The number of relationships increases with the number of people, so let's say this person had children. You would have to add them to the polymath, too.
It is not impossible, and it is quite a bit of information to process. It is hard, but it can be done if handled right. This forum is really good, and even if you take nothing from the threads, there are plenty of links to articles, websites that make for excellent reading material, etc. It is good to read about jealousy, time management, and anything that will affect you during this journey. Once again, I do wish you good luck. It is an interesting journey, and most would say it is rewarding. Feel free to ask questions if you stumble upon something of interest.