Then naked and white, all their bags left behind, They rise upon clouds and sport in the wind; (17-18)
The image of the children rising on clouds makes it kind of seem like they're on their way to Heaven. They are leaving their "bags" and clothes "behind," but the bags and clothes here might just be a symbol of their mortal life. They are leaving that behind and going to a better place. But are they really? What's this angel really offering them?
And the angel told Tom, if he'd be a good boy, He'd have God for his father, and never want joy. (19-20)
Okay, let's break this down. The angel's comment sounds nice enough. But upon closer look, it almost sounds like he's laying down the law. Does God only accept people who are good? And does being good mean Tom has to do his chimney-sweeping duty, as the last lines of the poem suggest? This doesn't quite sound fair. In order to be good and get to heaven, Tom has to slave away in other people's chimneys down on earth? These lines, especially when you combine them with the last lines of the poem, definitely make us suspicious of the angel's meaning.