Problem is, it's like vaccination (network effect). One kid with a hidden listening device could be in the vicinity of other kids whom it overhears, whose parents did not consent.
Which are just as illegal. The "ban" here is just the regulatory agency clarifying/informing the public that these products have been sold but are not legal to possess under existing laws in Germany.
This ban doesn't solve that problem though. A child could wear an adult smartwatch that has a listening capability.