Please ensure that your video and audio are enabled. Click on the padlock next to the site address in the search bar at the top of the page. This should allow you to 'view site information' and show you the settings for the camera and microphone. Please enable these settings. You may need to reload the page for the changes to take effect.
Check the settings on the headset or headphones that you child is using.
Tap on the microphone/mute button in the bottom left corner of the screen to ensure that the sound has not been accidentally muted.