Given that any computer system within the public sector is routinely derided (and not without good reason), I would never be confident enough to be sure that all the personal details associated with any fingerprint would be securely stored.
I am also a person who believes we are all too trusting in simply handing over our biometric details.
While to some people it may be an innocent way of ensuring books/meals go to the right person, the children themselves will get into the mindset of handing over their details to anyone asking who looks even vaguely official, whether or not that person actually has any right to ask.
The information I have at the moment is that the majority of those who suffer ID fraud are younger people. The very people who are MOST techno-savvy, who are the ones USED to handing their details around.
Older people like me tend to be more guarded with our information. If someone asks for something, my first response is always "WHY?"
For example, most banks ask for far more information about you than the money laundering laws require, and actually that extra information goes towards marketing products at you.
So, my dcs will not be getting permission to use a thumb print.