Sounds like you two have a pretty good relationship but now you both are a little apprehensive because you are approaching a mine field. Sorry to be so blunt. This comes from my own experience so, if it doesn't appear to match your situation, please just ignore this.
My ex went along with a little diaper play before we were married but she, too, assumed it was just a phase. Many years later the diaper thing was the major factor in our divorce. Don't underestimate her feelings about this. I believe that, over time, two main things need to be established.
One is that she needs to understand this is a permanent part of you that will always require some kind of outlet in order for you be happy. This doesn't mean there is nothing else in the relationship that makes you happy, just that if this aspect is ignored it will eventually strain your feelings because it simply will not go away. This may be a difficult thing for her to accept.
The second is that she needs to know you will always be the man in her life and will live up to her expectations as the man in the relationship. Whether her image is that of the traditional male role as lover, provider, and protector, or some other role model, she needs to know you want that too, and can handle it. Your baby side has needs that must be met one way or the other but but she needs to believe they will not interfere with your responsibilities.