If you wish to temporarily lock your child's card, you can do so directly via the YouTrip app.
1. Open your YouTrip app and switch to your child's account view
2. Tap 'Manage' and then 'Manage Card'
3. On the child's 'Card' tab, tap 'Lock'
Once locked by the parent, only the parent can unlock the card. If the child locks the card via their own app, they will be able to unlock it themselves.