Answer:
Because I could not stop for Death –
He kindly stopped for me –
and
Children strove
At Recess – in the Ring –
Explanation:
In the first stanza, the idea that death "kindly" stops for the narrator when she could not stop for it is paradoxical, because if she couldn't stop, that implies she didn't want to, yet she finds kindness in Death's unwanted overture.
It may seem paradoxical to say that children "strove" at recess "in the Ring" because it sounds as if they are fighting, not playing, which is the purpose of recess.