Most people's wishes are quite elaborate and complicated and tend to revolve around the acquisition of wealth and bright, shiny objects. But Tom Dacre, the little chimney-sweep of Blake's poem, has much more modest wishes. He just wants to get out into the open countryside and play with his fellow sweepers. There the boys will run down a green plain, leaping in the air and laughing as they go.
They will wash in a river and dry off in the sun. This may not sound like much of a wish, but we should bear in mind that as chimney-sweeps, these boys are covered in grime and filth most of the time and so for them getting a nice wash in a river is sheer luxury.
Tom Dacre's dream, in which his wishes are visualized, also contains a fantasy element. He and the other chimney-sweeps rise upon clouds and play among the wind. One can imagine that this would make a nice change from the soot and the dirt that they regularly breathe in as part of their work.
There's little doubt that Blake shared Tom Dacre's wishes. He was always highly critical about the appalling conditions in which Tom and so many other children like him were forced to live and work. He wanted children to be children, to play and to enjoy themselves out in the open air, instead of having to toil away each day in low-paid, dangerous, back-breaking work to survive.