The risks of employing a young student or the equivalent "nanny" aren't so much that they'll have malicious/criminal intent but far more likely that they aren't suited to the job or capable.
Most parents on here know how difficult it is to care for young children on your own, day in day out. It's relentless some days.
Add in the fact that children have been stuck at home during lockdown with no school for nearly six months, there is still little open (no soft play, pools etc) , it's unlikely the student will drive/ have a car, or have a network of other nannies to meet up with, or be able to pay for fun outings/treats.
Posters are suggesting these students can be paid minimum wage, no contract, no guarantee they won't walk from the job after a week. A job they perhaps thought would earn them some pocket money over the summer would quickly become the hardest, loneliest summer job ever. I can see a lot of them quitting or being neglectful of the children's health and/or safety as time goes on.
I have teens and although lovely and more than capable of a few hours babysitting would not be able to be a full time nanny to children for a summer. They're far too lazy and selfish for starters.
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It's understandable that parents might entertain this option because there is little alternative right now. They have to find a childcare solution.
It's also completely understandable for parents to have huge reservations and concerns about their children's welfare and the reliability of the teen they've got lying on their sofa scrolling Instagram looking after the dc.