One thing to keep in mind is that a lot of legal systems superstitiously assume that you can “guess” someone’s motivations, “intentions” (whatever this means) and similar. So cases like this often fall into a grey area, since they may or may not depend on the “intention” of who’s producing those scenes.
At least the UN definition defines CP as “any representation, by whatever means, of a child engaged in real or simulated explicit sexual activities or any representation of the sexual parts of a child for primarily sexual purposes.” I may be wrong but I do not think that the fetish itself counts as “explicit sexual activity”, nor that the target of the fetish (foot) counts as “sexual parts of a child”.