It is not a given that the mother will get the children. It boils down to who has been the primary caretaker and who will continue to provide the highest level of care and commitment to the children.
Continue to care for your family as it seems as though you have until now. One word of advice: records. Keep records of EVERYTHING. Even if the divorce is amicable, theings can turn nasty quick. Make sure you keep financial records, personal records so that in the event she does leave AND wants everything you are covered.