The work is believed to be worth about £500,000.
Golley Slater won the business, which is initially on a project basis, after a shoot-out with three undisclosed agencies. The pitch was managed in-house by Hamleys.
Agencies were briefed to encourage families to visit the store, amid mounting fears among retailers that parents would stop taking their children into central London following the terrorist attacks on the capital last month.
A Hamleys spokesman said the pitch process was ongoing and would not confirm the appointment.
Hamleys has traded for more than 200 years. With the support of the Icelandic retail giant Baugur, the Regent Street-based store delisted from the stock market in 2003 after nine years as a public company.
The Christmas campaign comes as specialist toyshops continue to lose market share. Recent Euromonitor figures showed a decline of almost 5 per cent since 1998 as supermarket toy sales increased.