I greatly appreciate your post and have three replies to your question.
First, I would emphasize that all this is new between you. Of course we all have associations with diapering for incontinence -- the young, the old, family members, the infirm, even ourselves. But adult-diaper play, though it comes in many permutations and combinations, is its own domain, with no clear-cut definitions or rules except those you choose to make and to follow and those we all abide. It is sui generis as a fetish, and it is unique for each couple. So see what works and how it evolves.
Many times diaper-lover play leads to adult-baby or adult-toddler play and back again. Some caregivers feel more comfortable if ab play is non-sexual while dl play is sexual. There need be no such hard divide between consenting adults, but it might help -- especially if switching occurs. (Some persons enjoy sexual diaper-play while being changed, but feel uncomfortable being sexual while doing the changing. Some feel uncomfortable if dl play remains sexual after changing to ab play, or if ab play turns sexual.)
It might merit emphasis that ab/dl play with a sexual component usually bears no relation to fantasies about relations or illegitimate conduct. Emphasize that the titles and roles of "Daddy" and "Mommy" have multiple connotations and aspects and profiles. It is rare that one wants one's actual mother or father to participate in absentia -- meaning one should be broad-minded and not take the roles too literally or seek to map out a one-to-one correspondence between actual parental care and ab/dl play. This of course is also true for parents as well as adult children -- when roleplaying with our adult significant other, as our adult other or as our little other, we are playing, not backwards-mapping roleplay, ageplay, or eroticism onto real-life relationships outside the bedroom, not recrafting or redrafting our actual relationships with our parents or our children, not paging Dr. Freud, and not involving others even in absentia.
Changing your wife's diapers might on the surface resemble to her at first your changing your daughter's diapers, but her point of view would literally be different (her lying on her back being diapered by you for the first time ever as a adult, I suppose, and loving every second of it, versus her standing up clothed and watching daily child-care given in a loving and appropriately parental manner), and should figuratively be different -- for your reasons for diapering are truly different, even though changing diapers always involves clean-up afterwards, of the changed child, spouse, parent, or incontinent.
Second, I would emphasize that whether together you enjoy ab play, dl play, roleplay, age play, something in between, or something else, it is adult play. Adult. That does not mean one cannot ageplay; on the contrary, it means you have great latitude to use your imaginations and to love each other as you choose. But ageplay has healthy and appropriate limits. And you know those limits, and you will agree upon them, and so you may take comfort in them and feel safe and healthy and happy within them.
Third, I would, if it seems right to do so, reiterate that ab/dl play is safe, sane, consensual adult play that need not contain elements you wish to exclude, and indeed frequently contains no traces of subjects and topics of a ghastly nature. Of course, sometimes a couple needs to play out certain scenarios in a therapeutic way -- intentionally evoking conduct or punishment, including that inflicted involuntarily, that one has suffered, including that borne long ago. That too happens from time to time, sometimes with healing results. But the divide between any ab/dl roleplay, ageplay, or other play and the realm of the illegal and the immoral is absolute.
I hope these answers are appropriate and helpful. Thank you very much for sharing your story and for posing your question. Good luck to you both.