In almost all cases I would have recommended to go Mont St. Michel. It is such a special and unique site. World heritage, of course, and so much closer than Paris.
Yet, in your case, as your children want to go there such dearly, it might be different: Even if you or they might go back in the future, if it is indeed their wish, it will be a special moment to see and climb the Eiffel Tower now.
They will probably treasure this moment.
Thus I second the idea of showing them pictures and cleary explaing them the effort needed to get to Paris, to make sure, it is really what they want and not just because it is the only place they have heard about.