Re: liability: definitely an issue. Might be the kind of thing that can only really be done in cooperation with eg ICMEC.
That being said, it might be possible to get to "better than nothing" with transfer learning, by training a model to detect both "there are children here" as well as "this is pornography". I have no idea what the success rate would be, but there have been some pretty impressive generalization results in recent years with classification models.