Hi there!
If you want to remove a saved Wi-Fi password on your child's iPhone, follow these steps from How to forget a Wi-Fi network on iPhone, iPad, or Mac:
Forget a network on iPhone or iPad
To forget the network that you're currently using:
1. Go to Settings > Wi-Fi.
2. Tap the info button
next to the network you're using.
3. Tap Forget This Network, then tap Forget to confirm.
To forget a network that you've joined before but aren't currently using:
1. Go to Settings > Wi-Fi.
2. Tap Edit in the corner of the screen to see the networks known to your device.
3. Take either of these actions:
* Tap the delete button
, then tap Delete.
* Or tap the info button
next to the network, tap Forget This Network, then tap Forget to confirm.
If you are trying to accomplish something else, please provide more specifics on why you are wanting to hide the Wi-Fi password so that we can continue to assist.
Cheers!