Ezra Taft Benson
Young mothers and fathers, with all my heart I counsel you not to postpone having your children, being co-creators with our Father in Heaven. Do not use the reasoning of the world, such as, “We’ll wait until we can better afford having children, until we are more secure, until John has completed his education, until he has a better-paying job, until we have a larger home, until we’ve obtained a few of the material conveniences,” and on and on.
This is the reasoning of the world, and is not pleasing in the sight of God. Mothers who enjoy good health, have your children and have them early. And, husbands, always be considerate of your wives in the bearing of children. Do not curtail the number of your children for personal or selfish reasons. Material possessions, social convenience, and so-called professional advantages are nothing compared to a righteous posterity. In the eternal perspective, children—not possessions, not position, not prestige—are our greatest jewels.
Brigham Young emphasized: “There are multitudes of pure and holy spirits waiting to take tabernacles, now what is our duty?—To prepare tabernacles for them; to take a course that will not tend to drive those spirits into the families of the wicked, where they will be trained in wickedness, debauchery, and every species of crime. It is the duty of every righteous man and woman to prepare tabernacles for all the spirits they can.”  Yes, blessed is the husband and wife who have a family of children. The deepest joys and blessings in life are associated with family, parenthood, and sacrifice. To have those sweet spirits come into the home is worth practically any sacrifice. 
Spencer W. Kimball
I have told tens of thousands of young folks that when they marry they should not wait for children until they have finished their schooling and financial desires. . . . They should live together normally and let the children come. . . . I know of no scriptures where an authorization is given to young wives to withhold their families and to go to work to put their husbands through school. There are thousands of husbands who have worked their own way through school and have reared families at the same time.