Although you make some good points, by applying it to the OP specifically you are risking jumping to judgement about someone else's family dynamics based on incomplete data. There are a lot of interelated factors here.Perhaps I should clarify myself more. I think there is a difference between teaching what the Word says on the topic and discussing bringing in another wife into the family with children. At least discussing when both spouses are not on board with the topic and it is a subject of marital contention. I just do not think that bringing in another wife should be discussed with young children until both spouses are in agreement and are certain this is what God is calling them too. If there is contention on the topic between spouses, we should not bring our children into it or any other contentious issue. Having read the OPs past post, it seems that his wife has not been on board with it until recently ( correct me if I am wrong). And the impression given is that this is something he has been pressing his wife for.
The general principles @steve has stated are sound. How these apply to the original poster and his family are for him and his wife to figure out.