The clamped-lips problem isn't a mystery. It's that none of the usual tricks solves what's actually in your way:
"Open biiiig!" gets you two seconds. The molars need ninety.
Even a cooperative kid can't hold their lips back and their mouth open while a brush pokes around. The lips creep back over the teeth within seconds — so you brush the front, guess at the back, and hope.
Flavored toothpaste and sticker charts bribe the mood, not the mouth.
Bubblegum paste gets them to the sink. It does nothing about the cheek that swallows the brush the moment you aim for the back teeth. You can't reward your way past soft tissue.
The singing timer apps assume you can see what you're doing.
Two minutes of brushing blind is just two minutes of blind. The problem was never the clock — it's that half the teeth are hidden behind lips and cheeks the entire time.
"Let them brush it themselves" ends with chewed bristles and clean-ish front teeth.
Dentists are clear that young kids don't have the dexterity to brush well alone — which is why cavities love the back molars and the gumline. The exact places you can't see or reach.
It isn't that your child is impossible.
It's that nothing you've tried actually moves the lips and cheeks out of the way — so every night you're brushing teeth you can't see.