There is this thread from 2019: Craft fair insurance for Jewellery
also this one about PLI from around a year ago: https://talk.folksy.com/t/public-liability-insurance-quote/27210
I don’t know whether it’s possible to buy insurance by the day but you could contact some of the companies that are recommended in those threads and see what they can offer. I hope your daughter finds what she needs and has a successful time at her fairs!
ETA did a quick google for ‘short term public and product liability insurance UK’ and the first thing that came up was this
One Off Event Insurance - Short Term Event Insurance | Protectivity
I have NO idea whether they’re any good but there must be lots of other companies offering one-off event insurance so that might be a productive line of enquiry 
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