Turns out when you enable Apple Card for a non-owner family member they have a max transaction limit of $250 but no total cap (can spend to owner credit limit). I thought the transaction limit would be much higher AND I thought there would be a total spending cap.
It's less useful than I expected.
Update: You can set a number for total spend notification and you can lock at any time. The $250 limit per transaction is a hard maximum though. Too low.