you can research on getting filters which can remove at least good part of fluoride, of course if you are in a situation where you can get clean water from other sources, its great, but most of us are not 
My son wouldnât let me brush his teeth at first either⌠he just clamped down on the toothbrush. But like many here he would let me put a finger in his mouth, so I used that little rubber fingertip brush for a long time. Now I let him wet the toothbrush, squeeze out the toothpaste and brush a bit and then I do all his teeth properly.
I always sing âBrush brush brush your teeth, brush the (food) awayâ to the tune of ârow your boatâ⌠and I name everything we had for dinner, or since he last brushed his teeth. It keeps his attention, and he lets me know if I missed something. It helps him understand why we are brushing his teeth, is a good food vocab review, and lets me brush long enough. Sometimes I slip in something silly like âbrush the mud awayâ or âbrush the shoes awayâ and he goes ânooooo!â and thinks it is funny.
Oh thatâs a great idea! we do a song too, but didnât think to include the things we ate that day. My son would love filling in those blanks (and of course responding to all the silly suggestions!).
He also really likes listening for the timer and telling me when to switch sides. I guess anything that really distracts them from the reality of brushing their teeth!
Oh, and I also found that the more âfunâ the toothbrush (cool pictures on it, or any cool shapes or whatever, like a car shaped handle) are absolutely terrible for my son. He spends the whole time wanting to SEE the car, trying to take the brush away from me so he can look at it. I give him a cool one to look at and play with, while I use a boring one to do the actual brushing with. I had one with a suction cup on the end, and though I CAN use it now, at first he just wanted to stick it to everything. He kept interrupting the brushing to stick it on things. Drove me crazy. 