In many discussions of the Daddy/little girl relationship, it is surprising how often some people have the idea tied to infantilization, or a perverse interest in underage girls. It is not viewed that way by those involved, but by others.
I have heard it said,
"At the extreme, the subs wear or are dressed in diapers and may use them, suck pacifiers and thumbs, play with dolls. They are bathed and tucked in. In less extreme cases, there is still an implied infantilization."
"They are sexually aroused by intimate contact with a little girl or possibly even with a daughter."
"The other motivation I imagine is that some daddy doms are relatively weak - and I don't mean that pejoratively. They have a dominant streak, but the only context in which they can safely express it is with a sub infantalized."
It has been my experience, both personally and by observation of others, that most Daddy and little girl relationships are not so different from other D/s relationships. He ties her up, he spanks her, she serves him, she gets release if he gives permission, and so on. Maybe the Daddy is older than his little girl, maybe quite a bit older. Maybe the Dom is more experienced or more mature or somehow has more authority or confidence; and the submissive defers to him. Often the overriding desires of the little girl are to be nurtured, and cared for and controlled, taught, and protected, perhaps different than the desires of other submissives and slaves who crave use and service.
But perhaps when he is Daddy it's more than just deference. It's more like he has a kind of infallibility. His authority is unchallengeable. It's not the same as the sheer power with which the slave-master asserts his will. The Daddy has a special aura of authority, in the face of which she becomes small, loses her grown-up faculties of self-determination and free will. In exchange she finds total security. I think in some cases, she is revisiting a relationship with an early authority figure in her life, perhaps one that never was completed or consummated.
It is a sense of "he knows what is best" for his little girl and she, knowing he has her best interests at heart, will obey her Daddy and feel comfortable and secure in "knowing" his love for her and the fact that she can please him by doing his bidding. It all sounds pretty much the same as most any submissive with her Dominant. I am not sure what the definitive difference is, often the more it is discussed, the more it sounds the same.