I agree an ankle worn signal jammer is easy to overcome with simple aluminum foil essentially putting it inside a Faraday cage. Further there is still wired connections but those are much easier for a parent/guardian to monitor.
The idea that the strapping of the jammer to ones legs is a health hazard must have already been addressed as the use for adult criminals appears to already be in place. That therefore is a non-issue.
I do question the statement that such a jammer is a violation of any type of right, human or otherwise. I do not know the laws in England, the UK, or Europe so please explain how a technology that did not exist when I was a child has become a right. Does this mean that driving a vehicle is a right? I can see a statement that one should have access to good free medical and dental, a warm dry place to live, water, and food as rights but to me, being able to place a telephone call or to text someone should at the highest be considered a privilege. A privilege that is paid for. Should it be a right, that would indicate the government should be providing the endpoint devices and providing the connectivity, for FREE, further there should be 100% fully powered coverage. Rights are free. Privileges can be purchased.
Hacking for purposes of criminal activity is a crime. It does not matter if the criminal is an adult or a child. Should they be disciplined the same? No. But both must be disciplined and the form of the discipline should fit the crime. I can make no recommendations on what forms as that will vary based on laws, beliefs, and attitudes within each jurisdiction. I do think it should be more effective than something overcome by a primitive Faraday cage.communication???