Human sexuality is not self-contained.
Behaviors and emotions that exist in a sexual context can be simultaneously juxtaposed with non-sexual elements, and vice versa. This is ultimately because the human brain is not a state machine; sexuality and non-sexual emotion are deeply intertwined because of their inherent thematic and functional relations as they exist in a neural network.
It's very attractive to say that this fetish is "sexual", and liken any partaking in it to be mere sating of sexual desire, because it's attractive (and trendy) to say that almost anything is grossly sexual, and embracing a fear often feels more cathartic than understanding the messy truth.
Which, of course, is that sexual desire is related to emotional desire, even though they are not the same, and both are capable of producing maladaptive behaviors.
Plenty of prepubescent children can exhibit maladaptive or strange behaviors (including this fetish) that will not be contextualized as a sexual fetish until they reach adolescence and/or incorporate the behaviors into their own arousal models. A child who engages in fetish activities is typically doing so purely for psychological reasons, not sexual ones (namely, it has been written into their attachment model) .
To that end, the ABDL fetish is not hard to unpack. Most commonly, it comes about as the "regression" defense mechanism as a response to parental neglect. Adult babies usually desire to be treated like an infant because, in their own tragic logic, such a state of being is among the only way to receive the love and care they need.
A child acting this way is really no different; they are not getting enough personal affection and attention, and now associate regression with affection (This can be amplified with other experiences, too). Usually, this is a problem systemic in the family, and cannot be resolved without drastic changes in familial behavior. Were I in this situation, hypothetically, it would mean I would have to commit to serious change of my own behaviors. I would re-evaluate how and when I show affection to my child, how receptive I am to their emotional needs, and work on building an association between progression and affection. I would be very careful not to demonize the fetish, because doing so would be to indirectly demonize love itself, and would create a tantalizing taboo for the child that would practically be cemented. In dealing with the topic, I would have to be very careful. Indifference and soft acceptance are probably the best options, as they place responsibility on the child, and create an emotional plateau, upon which I could condition them into more normal ways of seeking comfort and affection.
Ultimately though, there are much worse fetishes and psychological afflictions than being an ABDL. The real tragedy would be not getting what you need. That's what the brain seems to think, at least.