You could build your lesson around a story. Choose one that will interest the children and that is at the right level. Speak to their usual teacher who will be able to help you choose one they haven't already done.
Before starting, get them all sitting comfortably and in a place where they can all see you and the book. Get their attention and wait for silence before starting to read.
While you're reading, remember to engage the children in the story: mime actions, show them pictures and ask them questions.
When the story is finished, ask them some questions about it. Check with their usual teacher what level they are at so you can pitch the questions at the right level.
Then choose a language point from the story to develop. If someone goes shopping in the story, you could try a role play in a shop. If food is involved, you could teach food and food related vocabulary using pictures and / or props. You could also do a word search using some words from the story and develop the theme by adding other words. You could have the children copy some of the words, depending on their age and whether they are learning to read and write in English. You could also do a 'picture dictation' - take vocabulary from the story, read it out and ask the children to draw it. You could also take a theme from the story and do a song that is related.
Whatever you decide to do, check it fits in with what the usual class teacher is doing.
I'm sure you will enjoy it, teaching young children is very interesting!