I think there is a difference to being friendly and being friends.
If you want to continue to foster a relationship with you children and stepchildren just focus on being friendly. You have years of interacting ahead of you... grads, weddings, grandkids... If your children view you as unfriendly then they are constantly battling over having only one of you involved at a time.
Being friendly is being polite in each other's space. Right now if he is at home you are sharing the space.
Being friends is calling each other when the car breaks down or when there is stressful news at work to share. Is he calling you in the middle of the day to discuss work issues? Are you? Then I would say its safe to say you are not crossing the I don't want to be friends boundary.
If you can picture both of you at the same kid's event though with much distance between you and nothing more than a nice greeting... that's friendly. If you picture yourself at the kid's event siting next to each other and gossiping about the latest high school drama... that's being friends.
So if its more of the former... friendly then say nothing. Keep doing you.
