My situation is similar to the OP. DH and I were both widowed and married eight years ago. I have four children, 12 grandchildren and l & 1/2 great. He has two children, 11 grands and 7 great grands. There was never any issue with my children and grands - they all call him by his first name and they all seem to genuinely love him, not as a father, but as a respected older person in their lives. This is good.
When I first began to meet his grands, I made it clear to each of them individually, that I would always think of them as my bonus grandchildren, and the same for his great grands. I also assured them that they were free to think of me in any way they chose, and to call me whatever they felt comfortable with. It has worked out well, and so far, no bumps in the road. All of them call me by my first name, and I am very comfy with that.
You can't tell someone how to feel about you or what to call you. Just let nature take its course and accept each one of those "new" relatives as they come to you.