Like many poems in the Songs this is a complex exploration of the role of ideas and demonstrates that innocence and experience are one within the other.
The chimney sweeper who speaks first is, in fact, experienced. Distressed at Tom's suffering he cheers him with the false "never mind it". Tom is cheered and dreams a vision of escape which seems kindly and rewarding, but in fact is cruel for it keeps Tom locked obediently through "duty" in the cruel exploitative social evil of chimney sweeping.
The good angel is bad. But yet, paradoxically, their innocence is positive, making more bearable the pain from which, in reality, no one will release them.
The schoolboy complains that "school drives all joy away", that learning introduces destructive anxiety, sorrow and dismay. There is some truth in the argument. But the voice is not entirely genuine, it being too self-conscious, knowing, to be completely innocent.
The voice is also adult, and, rather, represents the dangerous sentimentalism of an adult who wishes to easily please a child, instilling false and superficial ideas in one too young to understand the value of education.
Thus the innocence or ignorance of this child will become later a prison of the mind. The simple unchallenged statements, the absence of questioning, show that this process has already begun.