Where is the child now?
You will need to inform the passport office that the child has returned to Moscow. It is likely that they will cancel your application as the child needs to be present in the country the application is made.
I hope this answers your questions. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.
You will have to request the return of the passport and cancel the current application. A new application for a passport would have to be made in Moscow. The attached form is quite clear and states , failing which, a new application for a passport should be made using the form on the following link: http://www.gov.ai/forms/passport/form-c2.pdf
If you did not correctly explain to the Passport Office that the child was in London, I suggest that you writ a clear and concise letter setting out all of the details.
I hope this helps
Do you have any correspondence from them detailing the passport number or even a copy of the old passport?
If so, this should be used to apply for a new passport in Moscow.