I met My Lady when her kids were older teens. Now they are 18 and 21.
We worked up to it slowly, and I tried very hard to meet them on neutral ground, not by invading their personal space.
The best laid plans don't always work.
I don't know what to tell you to do or say, but I prepared by telling myself to be aware they were sensitive to my presence, somewhat resentful (due to OW being forced into their lives), and mostly not to be pushy.
I will never be their parent; they have two of those. I try to respect their space and their place in their mother's heart, but I also need to assert myself. I, too, have a place in their mother's heart. And home. Which happens to be their home, as well.
It turns out, I actually like them. At least, on those rare occasions they choose to communicate. And they tolerate me. They apparently have little tolerance for fools, so I am taking that as a compliment.
At that age, they are no longer children. They are likely also not fully mature adults, in our current culture.
Be yourself. You don't have to win them over to you, they need to accept you as a part of their father's life. Hopefully, that can grow into more.