What's the Name of That Book??? discussion

This topic is about
The Dragon of Og
SOLVED: Children's/YA
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SOLVED. YA/children’s novella - read in 1993/1994 - unconventional princess, dragon/wyrm, plot involving gunpowder and sticky porridge, creation of toothbrushes. Possible Spoilers ahead. [s]
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I read this book in 1193/1994.
It was about a dragon who lives near a river, a princess who decides to save it from being killed.
One plan is to get it to eat sticky porridge with gunpowder mixed in to blow itsself up, however peasant children eat it all and the princess creates toothbrushes to clean their teeth.
In the end the dragon is decapitated, and thought dead but the princess places his head next to his body and he can heal himself - this is why he is called a wyrm
It seems to have elements of dealing with dragons, practical princess.
I’d love to find it