You can use the instant screen lock feature to lock your child's phone device remotely, so it becomes inaccessible. You can do it from the parent's app or online panel.
On the parent's app, from the Family tab click the Lock button next to the child's name you wish to lock. Type a message to display on the lock screen and select duration. Click Send and device will be instantly locked for the duration. You can unlock the device at any time by tapping the same button.
From the online panel, go to My Family page and tap on the Lock button next to the child whose device you wish to lock. Enter a message and select duration to lock the device instantly.