Is it permissible to await the completion of the iddah (waiting period) of the second divorced wife, and to keep her divorced in order to write a new marriage contract and mahr (dower), knowing that there is a child and the husband wishes to take her back?

We understand that your husband divorced you a second time and wants to take you back, but you refuse to return until the 'iddah (definition) (waiting period) expires and the divorce becomes irrevocable, after which he would عقد عليك عقداً جديداً (contract a new marriage with you).

If this is the case, then your husband has the right to take you back during the 'iddah, and you do not have the right to refuse the رجعة (return), for the رجعة does not depend on the wife's consent. Ibn Qudamah said: "The woman's consent is not considered in the رجعة, due to Allah's saying: {And their husbands have more right to take them back in that [period] if they want reconciliation} [Quran 2:228]."

However, if your husband agrees to forgo the رجعة until the 'iddah expires, and then contracts a new marriage with you, there is no sin upon either of you in that.