From what i have seen, people start off with restrictive boundaries and relax them over time. So just keep communicating so everyone knows where they stand and what they are confortable with.
As for custody, it probably varies by state. I think in general, it would be hard to take the kids away from their parents. From the cases I have heard, the worst made the third partner move out. Since you would be considered the third partner, I doubt that would be suggested. In general, I think you shouldn't have to worry about it. But you can err on the side of caution and not be explicit about what happens with anyone you think may frown on your relationship.