The last two lines of "the Chimney Sweeper" describe Tom's unrealistically dream about future. There is in fact an ironic tone going through those two lines. From the moment the sweepers were sold by their parents, Tom and his friends actually don't have many choices about their lives. By climbing up and squeezing themselves into the narrow and dark chimney to clean the soot, they are doing something that are not supposed to do in their young ages. Because of such enormous painful life, Tom could only put his hope into a dream. In Tom's dream, everything was sweet. They were doing things that they should have been doing at their ages, such as playing, leaping and also, washing their dirty bodies in the river. But the sweeter the dream is, the crueler the reality is being contrasted.
In the Poem, the image of God is merciful, nice and sweet. In Tom's dream, the boys could get anything they want, whereas those things would never be giving in the reality. In Tom's heart, God is a very important religious image for him. The author portrays an innocent and pure view of Tom, who truthfully believes that if "he'd be a good boy" (line 19), which means working hard without complains, he would "have God for his father" (20). When thinks about this, happy and warm arise from Tom's heart and strengthen his confidence of achieving his goal, which is stay with God. But is he really happy and warm? Getting up in the chilly morning and working in a dirty and black chimney will never make anybody happy. Warm and happy are just internal effects since the reality will not be changed after his dream. What is being changed is his spiritual belief; the belief that makes him deems he will go to heaven if he works hard.
The word duty in the poem is very important, because it reflects of the sweepers" harsh lives. The sweepers are usually being sold at age 5 or 6. What duties could kids have at this age? They are supposed to go to school and play around, but not work in the chimneys as child labors.