All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no harm in this if the sucking is intended for the purpose mentioned in the question. However, some conditions must be considered at the time of sucking: 1- that you and/or your wife are not sexually stimulated, 2- that you and/or your wife do not ejaculate, and 3- that nothing from the breasts reaches your throat. If any of this happens to you or your wife, then your action is prohibited, and if you or your wife experience a sexual discharge or something reaches the stomach of the husband, then the fast is invalidated.
Allaah Knows best.