Fear comes from many things
children pick up parents and families anxiety and fear
loss of control , new sensations can cause fear
fear of the unknown
the problem is each negative experience just reinforces the fear , even if nothing particularly unpleasant happened.
Don’t reinforce the negative (if you don’t have the filling the tooth will be painful and have to come out ) reinforce the positive (won’t you feel happy when this tooth is fixed) . This is a good overview of dental anxiety in children https://happykidsdental.co.uk/news/dentist-calm-nervous-children/
It’s important even if they don’t have the filling they keep going to keep them used to the dentist.
There are private dentists who will inhalation sedation for children many are in London eg Tooth beary if you Google paediatric dentist or dentist providing RA (relative analgesia) that may help to find someone, ask your dentist if they know a private provider.
In terms of diet you are looking at making sure that nothing sugary is eaten or drunk more than three times a day . It doesn’t matter what they eat or drink at meals times , but inbetween meals, even if it’s just a sip , or bite it has to be not sugary or starchy. So only plain water or milk inbetween meals , watch out for grazing on foods even things like yoghurts, granola bars , raisins , smoothies , fruit juice , biscuits , etc . Especially, even if they brush teeth afterwards, nothing to eat or drink just before bed apart from plain water or milk.