See the link below for another answer regarding this poem. Your question differs somewhat in that you are asking about the difference in themes between the octet and sestet of the sonnet.
In the octet, which occurs first, the poet introduces his simile - he is comparing nature to a "fond mother":
As a fond mother, when the day is o’er,
Leads by the hand her little child to bed,
Half willing, half reluctant to be led,
And leave his broken playthings on the floor,
Still gazing at them through the open door,
Nor wholly reassured and comforted
By promises of others in their stead,
Which, though more splendid, may not please him more;
He establishes the image of a child being led to bed, lovingly, by his mother. All children are reluctant to be so led and as the child is led, he gazes back at his "playthings" - wishing he could continue to play with them. His mother reassures him that he can play more the next day, perhaps even with better toys.
In the sestet, the author explains how the mother/child relationship is like nature/man:
So nature deals with us, and takes away
Our playthings one by one, and by the hand
Leads us to rest so gently, that we go
Scarce knowing if we wish to go or stay,
Being too full of sleep to understand
How far the unknown transcends the what we know.
Nature is our mother. She takes away our playthings - youth, health, love.........and leads us to "sleep" - probably death. And we don't even comprehend what the unknown is; we are too full of sleep. Perhaps we don't appreciate our playthings until we do not have them anymore.
The poem is really divided into two parts in this manner.
: H. W. Longfellow’s “Nature” is a fine example of Italian sonnet which has two parts, octave and sestet.
In the octave part of the poem Nature, the poet has presented nature as a loving mother and human being as a child. At the end of the day, the loving mother leads her half-willing and half reluctant child to bed with promises for more splendid play things instead of the broken ones. In the sestet part the same idea is reinforced. Nature, like a fond mother takes away all our earthly possessions one by one to prepare our mind for the ultimate rest, i.e., death. We cannot decide whether to go to the eternal world or stay in the temporal world. Finally, we surrender ourselves to the will of nature. Thus, the poem reflects an eternal truth and inevitable result of human life under the guise of a simple tale of a loving mother and her little child