This is my second marriage. No kids from first marriage. I often find myself thinking of my first Husband and wondering if things would have been different if we had children. Maybe we might still be together. Is it ok for me to feel like this?

It's very normal for you to think about your past. However, it has already a history and should not regret about your decision. Just move on and concentrate on current marriage. You can discuss with current husband on the baby planning and start working on it.