Heads-up: Xbox Live is on the blink today. The official status page notes that "Xbox Live Core Services" may be "limited" - in particular, you might have trouble signing into a child account. The Xbox 360 is the only console affected.
"Are you having issues signing into a child account? Our team is actively engaged and investigating this issue," reads an update from about 5pm yesterday. "In the meantime, try visiting http://support.xbox.com/en-GB/contact-us?isChatCallAvailable=false, which will provide you with the contact information to help you with a temporary work around for the issue. Thanks for all your patience. We will update you as soon we have more information."
Permit me to save you the labour of clicking through to that Support page by reposting the contact details below. I know, I'm a regular diamond in the rough.
Phone assistance is available.
020 7365 9792 or 0800 587 1102
Hearing Impaired (TTY device): 0800 5871103
Please note that if you call us using a mobile phone, there may be additional charges from your service provider. Please contact your service provider for more information.