‘The Child is the father of the Man’ is a line from Wordsworth’s-

My Heart Leaps Up (Also called The Rainbow)
by William Wordsworth

My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
And I could wish my days to be
Bound each to each by natural piety.