Is a mother's leaving the prayer due to her child's need to use the bathroom considered justifiable, and does the husband receive the reward of congregational prayer in this case?

You will receive the reward for congregational prayer by leading your wife, because the virtue of the congregation is attained by two or more people. However, your wife's prayer is invalidated by her leaving to take the child to the bathroom, due to her many actions that are contrary to prayer. Her intention to interrupt the prayer, if leaving it would result in a legally recognized harm, such as harm to the child, is considered a legitimate justification. You should complete your prayer individually, and it is valid, and you will receive the reward for congregational prayer, Allah willing, as long as you were keen on it and then something prevented it.