Would my action be considered unjust or would I be sinful due to my desire to take back my first wife for the sake of the children after my engagement to another woman and her refusal of that?

Scholars have differed on the ruling regarding fulfilling a promise, and the preponderant opinion is that it is recommended (mustahabb) and not obligatory (wajib). Therefore, there is no sin upon you for not fulfilling this promise and for taking your wife back, and you are not considered unjust to her. If a benefit becomes apparent in taking your wife back, then you should explain this to your fiancée in a kind manner, considering her feelings. One should be cautious and not hasten to divorce, due to its negative effects, especially on children.