I have a UK child visitor visa but I have to transit through the UK from USA. Is this possible if I'm going alone?
A UK Child Visitor Visa is just like a normal visa except it contains an endorsement which identifies a parent/guardian. If the parent/guardian is not present and accompanying the child, then it's the same as the child not having a visa at all.
So the child would be seeking to transit under the transit without visa rules. It means the carrier would not allow boarding. Were the child to reach the UK they would likely be detained under the provisions for child protection.
The UK authorities would be very upset because a violation under Paragraph 320 had occurred. The credibility of of the accompanying parent would be shot to hell and their entry clearance would be revoked and they wouldn't be able to get visas for a while.
Please also see (for the legal references): Children with a different surname to parent - UK citizen - UK passport control