A security researcher found the devices neither encrypted the data they used nor secured each child's account.
As a result, he said, he could track children's movements, surreptitiously listen in to their activities and make spoof calls to the watches that appeared to be from parents.
Experts say the issues are so severe that the product should be discarded.
Both the BBC and the researcher involved tried to contact the makers of the MiSafes Kid's Watcher Plus to alert them to the problem but received no reply.
Likewise, a China-based company listed as the product's supplier did not respond to requests.
The Norwegian Consumer Council highlighted
other cases of child-targeted smartwatches with security flaws last year .
It said the MiSafes products appeared to be "even more problematic" than the examples it had flagged.
"This is another example of unsecure products that should never have reached the market," said Gro Mette Moen, the watchdog's acting director of digital services.
"Our advice is to refrain from buying these smartwatches until the sellers can prove that their features and security standards are satisfactory."
In the UK, Amazon used to sell the watches but has not had stock for some time.
The BBC found three listings for the watches on eBay earlier this week but the online marketplace said it had since removed them on the grounds of an existing ban on equipment that could be used to spy on people's activities without their knowledge.
"We don't allow the sale of these products on our marketplace," said a spokeswoman.
MiSafes previously made headlines in February when an Austrian cyber-security company discovered
several flaws with its Mi-Cam baby monitors .
SEC Consult said these meant hackers could spy on footage from owners' homes and hijack accounts.
It too was unable to get a response from the manufacturer.