I lead a lot of writing workshops for kids and sometimes the will is there, all raring to go, but the well is dry, so to speak.
My writing teacher- the amazing novelist and memoirist Abigail Thomas- used to give us topics in our weekly group meeting which we could run with or not, and I’ve adopted her methods, slightly altered to be more kid friendly(read- funnier and/or vaguely gross).
So here are a few ideas for writing subjects if you’re feeling stuck. I’ve used many of them myself.

A paragraph about your morning using only three word sentences- not two words, not four, but three- no more, no less.
Example- (Off to school. I’m not happy. Big math test. I hate math. I fail miserably. Teacher not pleased. Peanut butter lunch. Late soccer practice. Back at home. Mum cooks stew. Apple blueberry pie. Makes everything better. feel pretty good. Mum explains algebra. I finally understand. Dream good dreams.)

A paragraph about an unpleasant job using three-word sentences.

A paragraph containing a dog, a banana and a thunderstorm.

A paragraph containing a wizard, a tea kettle and an uninvited guest.

A paragraph containing a bowl of cereal, a hand-knitted scarf and an angry relative.

A paragraph containing a pair of scissors, a work of art and a broken window.

A paragraph containing a panda bear, a bouquet of flowers and a very short man.

A paragraph ending with the words ‘The smell lasted forever.’

A paragraph beginning with the sentence’He didn’t like sea otters.’

A paragraph ending with the sentence ‘So finally it was over.’

A paragraph beginning with the words’A chill went up my spine’

A paragraph about cheese
A paragraph about being a kid
A paragraph about doing nothing
A paragraph about the sky
A paragraph about snails