What is the main idea of the poem "We are Seven"?

The main idea of Wordsworth's poem "We are Seven" is that the dead still "count", even though they are no longer with us. In the poem, a man approaches an eight-year-old girl and asks her how many brothers and sisters she has. She answers that "we are seven" (meaning there are seven siblings total), and proceeds to explain that two of her siblings lie in the church yard. She further details how first her sister, Jane, and then her brother, John, fell ill and were laid to rest in the church graveyard, not far from her home.

The man counters the girl, stating that, as only five siblings are still alive, her count should be at five. He contrasts the siblings whose souls "are in heaven" with the girl, who has limbs that "are alive". She counters with tales of how she still interacts with her dead siblings, visiting their graves often, singing, etc. In the end, as the poet asserts, the "little maid has her will", as she will not be convinced that she has any less than a family of seven.  This is one of those cases where children can be more knowing than adults, as most readers will find themselves siding with the girl.