I'm sorry but tooth brushing is only part of the problem. If you read links above the more times a day you snack , the more likely you are to get decay . Here's a quote
"Excess sugar drastically increases the chances of tooth decay, cavities, and tooth loss. A report published by University College London in 2014 is the most widely accepted article connecting sugar intake amounts and dental health.Teeth are more susceptible to decay immediately after first breaking through the gums. If a child aged 7-10 sticks to the 24 grams of sugar a day limit, they could see a 10% chance of developing tooth decay in an adult tooth that’s been erupted for a year. If this is pushed to 40 grams a day, this becomes a 25% chance - a huge increase. On average, for every increase of 25g/d in sugar, one tooth in a child will become decayed, missing or filled. Any damage caused can’t be reversed, and will only accumulate, resulting in further dental damage and costs later in life."
We make choices for our children all the time and particularly when dental care can be difficult to access making the right choices is even more important. Tooth brushing alone will not stop decay , if someone has sufficient number of sugar attacks during a day then they will get decay whether they brush or not. Brushing with a fluoride toothpaste reduces some of the risk and damage but not all. It's important to be informed to be giving the healthiest choices if you decide to give your child snacks.
You can eat nothing but chocolate for breakfast,lunch and dinner with no snacks /sweet drinks in between and whilst it would be dreadful for your health you probably wouldn't get decay.
However eat a normal breakfast , lunch and dinner with small snacks of flavoured yoghurt , fruit juice, fromage frais frequently during the day or anything that contains sugar and you will most likely get decay.
The sugar in sweets is no worse than the sugar in granola bars or flavoured fromage frais , it's still a sugar attack that will take an hour for the tooth to recover from.
Unfortunately the problem of tooth decay has an awful lot to do with where you live , a child is 20 times more likely to have decay in the worst area of England compared to the best. It correlates strongly to social deprivation. The reasons are multi factorial but the consumption of sugar and frequency is one of the main reasons. This is why understanding what the most effective things you can do at home is so important . These are , as you say , brushing twice a day with a fluoride toothpaste and spit , don't rinse. But equally keep the number of sugar attacks a day to 3 to 4 . As every meal will have sugar in that means if you give snacks and drinks in between meals make them free from sugar and try to keep foods and drinks with sugar in to meal times only.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/768368/NDEP_for_England_OH_Survey_5yr_2017_Report.pdf