As a general idea the Mishna Brurah writes that the idea of fasts is to bring one to do tshuva not to just fast and go on like a regular day,it is like taking the tafel instead of the ikar. If a mother needs to nurse then she should eat what she needs. If one is allowed to eat it means just that but to iteat fancy things or sweets that are not necessary does not fit with the spirit of the day.