This was posted in this subforum recently on this same topic:
The following are excused from the law of abstinence:
"Children under the age of seven, those who have of come to the use of reason, those who cannot without grave inconvenience get abstinence fare, those who cannot abstain without considerable difficulty, whether of health, time, expense, the sick, the convalescent, the very poor, those who require meat foods for the sake of health or work, those engaged in hard manual work, wives and children, who, by abstaining, would incur the serious anger of the father of the family, vagrants who live on what they can beg, members of the armed forces during service, that is, so long as the are members."
From: Moral and Pastoral Theology, Davis, 1952, p. 125.