earlygirl I am a dentist and can assure you 100% that the decay reducing property of brushing is down to it as a means of fluoride application . Actually there is an exception to this - brushing even without any paste will reduce decay on smooth surfaces of teeth but these are not the ones that are the most likely to decay. It's the interproximal ( where two adjacent teeth touch) and occlusal(biting) surfaces that are likely to decay and I repeat you might as well not bother brushing at all as use non fluoride paste.
of course it's all about diet and your oral bugs really!
Yes Nutty, from as soon as they have teeth, but just a tiny amount and don't let them have free access to the tube. Get them to spit out any foamy residue.
Look for 1000ppm fluoride on the label. Some kids pastes have this, but many are only 600ppm.
My kids are like yours, thay always moan it is too minty. I sometimes buy Punch and Judy strawberry which they like best but it is expensive (especially as I get other minty stuff free)