This is from the preface of Isaac Watts' Songs for Children (1715). I especially like the image in #3. A song is like constant furniture in the mind of a child. It is always sitting there in the room of the mind--welcoming a child to come and recline when they are at leisure.
Verse was at first designed for the service of God, though it hath been wretchedly abused since. The ancients, among the Jews and the Heathens, taught their children and disciples the precepts of morality and worship in verse. The children of Israel were commanded to learn the words of the song of Moses, Deut. xxxi 19, 50, and we are directed in the New Testament, not only to sing "with grace in the heart, but to teach and admonish one another by hymns and songs," Ephes. v. 19. And there are these four advantages in it.
I. There is a great delight in the very learning of truths and duties this way. There is something so amusing and entertaining in rhymes and metre, that will incline children to make this part of their business a diversion. And you may turn their very duty into a reward, by giving them the privilege of learning one of these songs every week, if they fulfil the business of the week well, and promising them the book itself, when they hare learnt ten or twenty songs out of it.
II. What is learnt in verse is longer retained in memory, and sooner recollected. The like sounds, and the like number of syllables, exceedingly assist the remembrance. And it may often happen, that the end of a song running in the mind may be an effectual means to keep off some temptations, or to incline to some duty, when a word of scripture is not upon their thoughts.
III. This will be a constant furniture of the minds of children, that they may have something to think upon when alone, and sing over to themselves. This may sometimes give their thoughts a divine turn, and raise a young meditation. Thus they will not be forced to seek relief for an emptiness of mind, out of the loose and dangerous sonnets of the age.
IV. These Divine Songs may be a pleasant and proper matter for their daily or weekly worship, to sing one in the family, at such time as the parents or governors shall appoint; and therefore I have confined the verse to the most usual psalm tunes.