Hey there Hi there Hello there saudade
My wife and I were married in a non ploy relationship before we meet or Sweet sweet nikki. But had we known then what we know now what you are doing sounds exactly the way we would handle it. My wife and I have talked about how we can give nikki a felling of having stock in our impending relationship. I too think the best thing to do is get a lawyer to to draw up some legal doc to to make your "not so marriage marriage" actually have a leg to stand on.
Cool thing about my situation is that we all already have the same last name
hope it all works out for you