The child key itself. The server keeps only its hash, so it cannot show it again.
When the child stops working, as an ISO 8601 time.
Most the child can spend from the parent's balance. null means no limit beyond it.
The parent's credit balance when the child was made, in USD. The child spends from it.
The child's per-minute spending cap, in USD. null means none.
The answer from POST /key/child: a new child key, returned this once.