There is this poem I knew when was little something about Belinda who had a dragon called Custard to do with Mustard and weelbarrow What it?

The poem you're thinking of is "Belinda and Custard the Dragon" by Helen Cresswell. Here it is:

Belinda and Custard the Dragon

Belinda lived in a fairy glen

With her great big dragon, Custard.

They were the best of friends and when

They played together it was just grand.

One day, they went to the wood to play,

And found a wheelbarrow full of hay.

Custard jumped in, and Belinda too

And off they went, a jolly crew.

They bumped along until they came

To a deep and muddy, marshy place;

And oh dear me! they did not know

What to do, with that silly face!

Then Custard had a splendid thought,

And gave Belinda quite a fright,

He put his arms around the hay

And with a puff blew with his might.

He blew and blew, and puffed and puffed,

Until the hay flew to the sky

Then down it came and filled the mud,

And made a path for them, oh my!

So Custard saved the day, you see,

When they were stuck in that muddy place,

And Belinda gave her dragon, glee,

A special hug, and kiss on the face.