The Zentra disguised the Zentrabots as a "toy craze" over the Christmas holidays on Earth. On Christmas morning, an activation signal was sent and hundred thousand children on Earth were taken hostage on board the Zentra mothership so that the governments on Earth could hand the resource-starved Zentra their weapons.
After the Eleventh Doctor tracked the signal to the mothership, once on board, Danny rammed into a Zentrabot on his skateboard. The Doctor captured it and programmed it to reprogram the other Zentrabots to respond to a signal to turn against the Zentra guards. The Doctor successfully emmitted this signal and rescued the children held hostage. (COMIC: The Zentrabot Invasion)