If this is the case, you should talk to your pediatrician about simple, natural ways to soften her stool. However, I have found that many infants seem to fuss even in passing very normal, soft and even loose stools. Generally, this fussiness goes away by about three months of age. This is a complex question, which I can certainly not answer completely here. First and foremost, it is so important that your children know that they are not at fault in this situation.
Most children are naturally egocentric and feel that all family problems are somehow their fault.
They must know that this is not true and that both parents love them dearly. I strongly recommend a book called Divorce Casualties to both you and your ex. It is available online and in most bookstores. This is the best book I have seen to help parents avoid making their children the victims of a failed marriage. Most of all, be honest with your children about your feelings for them.