There's a massive gulf between conducting a thought experiment and conducting applied research that involves human subjects. Beyond that, though, there is IRB-approved research dealing with adult-child sexual relationships. This is not my area of expertise by any stretch of the imagination, but I encountered some of this research in my doctoral coursework only a few years ago.
The human subjects research I encountered principally involved interviews with adults about their experiences as children and/or minors or with people convicted of statutory rape and related crimes. This research tended to revolve around borderline cases, and not cases involving very young children, but it was certainly done with IRB approval. Many of the researchers advocated reforms to age of consent laws, the abolition of sex offender registries, and the like. I disagreed with some of their conclusions, but I would not call their research unethical.

I see no reason why moral philosophers should be effectively prohibited from probing some of the questions this kind of research raises even further. I don't particularly like it, but I don't think the questions are invalid, either.