Kids YouTube comes under the same category as Adult YouTube, so the access should be the same.
To enable us to investigate into the issue further, could you please provide the following:
1. The user of the device
2. What device is experiencing these issues
3. What time and day they were unable to access it
If you feel like your child's "Internet age" is older than his actual age you can manually move him to an older age bracket by clicking on his profile in Portal and selecting the Settings option. You can also deviate from the standard settings in his actual age bracket by toggling any of the controls in Portal > Cyber Safety or Zone Manager > Settings.