"Psssst, kids, I'll tell you a secret: there are coins lying around at home. In bottles and in jars," comes the conspiratorial whisper on the television. "Look for them and give them to your mum or dad. Because when the euro comes they'll have to change them anyway."

The commercial, starring a popular entertainer who presents the German version of Who Wants to Be A Millionaire?, is part of a campaign by the Bundesbank to enlist the nation's children in preparations for the introduction of euro notes and coins in just over six months. It is targeted at what the bank has dubbed the Schlafmünzen, or "sleepy coins", that slumber undisturbed in piggy banks, kitchen drawers, jam jars and down the sides of sofas.

The effort and expense of the campaign, with the slogan "Bring out those sleepy coins", is justified by the scale of the task facing the authorities. Germany will have to collect 28.5bn D-Mark and pfennig coins weighing 100,000 tonnes as part of the changeover.

Of these, it is estimated that 6bn-10bn, weighing 30,000 tonnes, are Schlafmünzen targeted by the campaign. The Bundesbank wants as many of these as possible to be paid into banks well ahead of time, rather than have queues of people with bags full of petty change in the first weeks of next year.

"Our aim is to get at those coins that are currently just lying around gathering dust. We want to get those out of circulation now, so that when the crunch time comes in January we can concentrate on the remaining cash that is being actively used," says Gabriele Reitz-Werner at the Bundesbank.

Mr Reitz-Werner will be ably supported in this by a cast of cartoon D-Mark coins that made their appearance in numerous kids' magazines throughout May and continue to inhabit a special website featuring kids' games and a quiz. The hoarded coins can be left in 20m "safe bags" at bank branches throughout the country, to be sorted, counted and then credited to the individual's account.

"We've seen people with everything from briefcases, to panda bear bags and bottles, mostly with very small-denomination coins," says Armin Czysz, who heads an information campaign tour of 100 German cities until the autumn that also offers a coin collection service.

However, Germany's main banks say that the reaction at their branches has been muted so far. HypoVereinsbank, for example, has seen only a slight increase from normal levels. It plans to keep the momentum going by stepping up reminders to its customers through mailings, messages on account statements and collection boxes at its branches.

To date, says Manfred Weber, the head of the German federal banking association, "the piggy bank campaign has not been as successful as we had hoped. Far fewer than the targeted 8bn-10bn coins have found their way into banks."

The campaign to erode the mountain of Schlafmünzen, it argues, may have to be resumed in the autumn if chaos at the tills is to be avoided early next year.