I think there is a possibility. He went to great lengths to explain vows and restrictions, and talked about taking the risk to leave the choices for the participant to make the ritual fail. A baby cannot do anything related to consent or choices, and there might be a loophole regarding that.
My guess is the mother acts as a stand-in for any princes too young to fully participate. Though it's an interesting thought. It might also be a lie/ruse by Kurapika, and the ritual automatically signs up everyone who meets the selection criteria.