Start by introducing the idea of a clerihew with examples. The one above is by the poet Edward Bentley Clerihew himself, but you can easily find other child-friendly examples on the internet, or knock off a couple yourself. Explain that although the rhymes have to be a,a, b,b, the lines can be any length (they don't have to scan in the traditional sense).
I wrote one on the board about myself:
The trouble with Mrs Long
Is that she's never wrong
She'll put up a fight
To prove she's right.
Then give out one or both the sheets of possible starting lines and write a few together.
You can also issue the children with a sheet of easy rhymes.
The students found it straightforward to write in the clerihew form, but extremely tricky to get the last lines right! The main aspect of Writing Club, though, is to have fun and to experiment, and today's session was all about that. I do think, too, it brought home how clever and skillful good rhyming poetry really is.