It's a fetish, and it has a sexual aspect to it. They pretend to be infants, act like infants, dress like infants, and this turns them on (and their partners). While it does not directly involve children, there's an element to it that's very pedophilic. It's definitely a very disturbing and uncomfortable thing to be into. It's deviant and perverted, that's for sure.
Maybe it's
Peter Pan syndrome... Michael Jackson had it. He said: "I am Peter Pan in my heart". Refusal to grow up, being a "child at heart". Not the type of people you'd be comfortable having your children around, because there's always that fear. If you see yourself as a child, bounderies of age kind of don't exist in your mind. It's not hard to see how this can be very problematic.
There are elements of pedophilia involved with this fetish that are undeniably there. The fact that the fantasy involves babies, who are the most helpless and vulnerable of all people, gives me the absolute creeps. Clearly these people are mentally ill.