To "cram" is to force something into a space that is too small, for example a tourist cramming her souvenirs into her suitcase at the end of her vacation. It's also used to describe a student studying for an exam at the last minute. In your example, working mothers are trying to do all the things they imagine stay-at-home mothers are doing (for example, taking a daughter to dance class, cooking a special meal for the family), but they don't have the time to do it, so they end up "cramming" these activities into a much shorter amount of time. "Cram" always implies that it's a tight fit!

How I understand this is working mothers try to do all the things that a stay-at-home mother does (cooking, cleaning, etc) into the evenings when they get home from work. But because they try to do too much, they get too stressed and it can be hard on the whole family.