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 is His slave and Messenger.
The matter which you raised in your question is easy and it should not cause a dissension between you and your husband. Islamically, both the mother of the father and the mother of the mother are considered as mothers for their grandchildren, Allaah Says (what means): {Prohibited to you [for marriage] are your mothers.}[Quran 4:23] The term mother includes the mother of the mother and the mother of the father and it even includes the great-grandmothers as interpreted by the scholars .
Therefore, it is permissible for the grandchildren to call their grandmother ‘mom’ (mother).
Allaah Knows best.