Rinsing with peroxide has helped my teeth along with some tea tree oil, might be hard to get a 5 year old to do this though. Your saliva should be full of minerals to remineralize teeth but first one must kill the bacteria.
There are also a few rife frequencies that target mouth pathogens but this requires purchasing a rife machine.
A dental water pick works well with a salt solution, tea tree oil, peppermint oil etc. Even a tablespoon of bleach in the pick will kill bacteria as well in the hard to reach places.