Please ensure that your video and audio are enabled on your browser on your device. To do this, click on the padlock next to the site address in the search bar of your browser. This should show you the settings for the camera and microphone. Please enable these settings by toggling the camera and microphone setting to the blue 'on' position. 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.