By the end of the year, children in Reception should … "use their phonic knowledge to write words in ways which match their spoken sounds. They also write some irregular common words. They write simple sentences which can be read by themselves and others. Some words are spelt correctly and others are phonetically plausible."
To support simple sentence writing, children can look at the pictures on the document below and write the word. They can then build this into a caption and then a sentence.
You could choose two pictures to build your own silly sentence. E.g. a cat in jam.