why are my daughters so prone to cavities?

Unfortunately the answer may well be in your question. Scotland has just about the worst record for dental cavities in the developed world. This is a lot to do with the resistance to adding fluoride to the water in some areas. If you are in one of these areas ask your dentist about fluoride tablets, Just one word of waring though it is important to get the balance right between fluoride in the water, fluoride toothpaste and fluoride tablets as overdosing marks the teeth.

Water percentis no longer another choice to flossing. It actually doesnt do lots to assist. She would desire to brush three times an afternoon (AFTER nutrients) and floss as quickly as an afternoon. whilst she brushes she would desire to brush for 2 minutes each time.

have you tried another dentist to get a second opinion

I think your kids need to floss. You didn't mention if they did or didn't, but flossing could help with the cavity problem. Flossing seems to be the only thing I can think of.

Whether you receive cavities has a lot to do with genetics regarding the density of the teeth. Some people's teeth are more "hollow" than others and break easier and get cavities easier, and if you're that way, well, you're more prone than others to get cavities, like it or not. Beyond that, what you eat, some of which may chip your teeth, like ice or popcorn kernels, are things you need to stay away from. Also, be sure your children are flossing. Many people don't like to take the time to floss their teeth, and they think if they just brush harder or more often or if they use one of the hundreds of mouth washes that promise this and that, that they won't need to floss. None of the aforementioned dental health care will remove the need to floss. I know there is products advertised on television and a lot of "substitute" things that promise that you don't need to floss if you just do what they say.

Also, if your children eat a lot of fruit it may also be affecting the cavities in their mouth. Fruit is essentially made of glucose, fructose, and sucrose (sugars) and too much intake of the fruit may be affecting their teeth. Tooth decay is caused by specific types of acid-producing bacteria that cause damage in the presence of fermentable carbohydrates such as sucrose, fructose, and glucose. The mineral content of teeth is sensitive to increases in acidity from the production of lactic acid. Specifically, a tooth (which is primarily mineral in content) is in a constant state of back-and-forth demineralization and remineralization between the tooth and surrounding saliva. When the pH at the surface of the tooth drops below 5.5, demineralization proceeds faster than remineralization (meaning that there is a net loss of mineral structure on the tooth's surface).I mentioned earlier that if they are not flossing more cavities are likely to form. If your children do not floss already I suspect that the sugar is getting down in between their teeth and it is not being removed with a toothbrush causing plaque buildup and later cavities. I'll list some helpful advice to help correct their problem:
1.) use a toothpaste that has a fluoride in it
2.) brush after 30 minutes of eating; if you brush immediately after you eat the enamel will remain soft and be brushed away causing more cavities to form
3.) floss after brushing
4.) use an anti-cavity mouthwash

Also, just as a side note. If you received cavities after using mouthwash you should talk to your dentist. There may be a genetic flaw with your teeth that needs special attention.

It may be because they eat a lot of fruit. Fruit contains acid which wears down the enamel on your kids teeth. It does seem really strange that they're getting so many cavities. Are they flossing often and using mouthwash as well as brushing?

It's really strange that your teeth rotted after using mouthwash, you should talk to your dentist about that.

Make sure they aren't brushing their teeth straight after eating because this will brush away the softened enamel on their teeth. They should wait at least 30 minutes.

Fruit also contains a lot of natural sugars. Try replacing some of the fruit with vegetables instead, like carrots and celery. Use a toothpaste with fluoride in it. It should say on tha package if it contains fluoride.

If you're really concerned your dentist isn't doing his job properly then go to a different dentist.

That's quite odd. Do any other relatives have dental problems? If so then you may just unfortunately have a gene that contributes to weak teeth. If not, then I'd strongly recommend finding another dentist online and trying to contact them, as yours seems to be very unhelpful.
Have you tried changing your toothpaste and toothbrushes?
Some people are just more prone to cavities than others. But you should definitely get a second opinion.