I don't think the dentist is fleecing you. My DD has poor enamel and has a small filling (not metal) on a front tooth. Your child will have her baby teeth for years and their health and placement affects her permanent teeth.
Right now, it's a small thing. Later it can be a big thing. If you wait til she's in pain, you're waiting for a potential abscess, or even a systemic infection. Google for "death toothache" and you will find stories, including that of a 12 yr old boy, who died from infections caused by untreated dental issues. That's an extreme case, but it happens. I know someone whose son had to be put under major anesthesia and have caps and other serious repairs done on his teeth because it wasn't caught or treated earlier. That's what cleanings and routine visits are for - to catch things when they are small and fixable.
I do not regret getting my DD's tooth repaired (it was so small they didn't need to drill at all) and I had my teeth fixed when I was a kid, too.
You may have to take additional steps to prevent more cavities, but I would fix the ones she has.
You can also request that the dentist use the other fillings and you pay out of pocket.
Teeth that touch need to be flossed as those are areas that are hard to reach and common for cavities. If she doesn't yet floss, teach her how and/or follow up. Til children are 6 or sometimes older, they need parents to check up on or brush and floss their teeth to ensure good habits.
Instill the habits now. My stepson is terrific and has had zero cavities his whole life. His sister is a sugar junkie with meh habits and last year it was about $2k to fix the cavities she had. We were not pleased.