Answer

You can say anything. Is there a prohibition against causing a minor to sin? He is not halakhically obligated. Quite a few halakhic decisors wrote that spiritual damage is caused only when there is an actual transgression in the act itself (especially regarding a nursing infant in Ketubot 60 and in the Shulchan Arukh concerning nursing from a non-Jewish woman).
In any case, this depends on the question whether the prohibition of causing someone to stumble is because of the spiritual damage. It is certainly possible that even if there is spiritual damage, there is still reason not to cause it, but that still does not mean there is a prohibition of “do not place a stumbling block” here. Think about the case of one side of the river. From the perspective of damage to the world, I would have expected this to be prohibited just like the case of two sides of the river (though my responsibility is less there, I am still with my own hands creating damage in the world). And the same applies to “let the wicked wallow in it and die”: from the standpoint of damage to the world, there is no room to distinguish between a wicked person and a righteous one.