The quotation used as a preface to the novel "Silas Marner", by George Eliot is...
"A child more than all other gifts
That earth can offer to a declining man
Brings hope with it, and forward looking thoughts"
However, the quotation is not from William Wordworth's poem "We are Seven" as stated in the quaestion. The quotation is from Wordsworth's poem "Michael: A Pastoral Poem"... here is the quote exactly...
"Than that a child, more than all other gifts
That earth can offer to declining man,
Brings hope with it, and forward-looking thoughts,"
This line from Wordsworth conveys that as we grow old and decline in vigor in this physical life, there is hope because of the continuation of our family line through our children. This quotation also conveys that our children can be a source of joy and hope for the future as they embark on their life journey with the ideals of energetic youth firmly in place.