Is there a person specification? They usually help to show what they are looking for. I would write a paragraph to state what job you are applying for and why.
Personalise it to the nursery so find out some information about it and say that you want to apply for that particular nursery because ..... Then include a paragraph detailing your experience and how it is relevant to this particular job.
Another paragraph could be talking about any hobbies and interests you have that are relevant to the post and maybe could help you plan certain activities with the children.
Also include a paragraph talking about professional attributes ie good team worker, always punctual and very reliable, willing to take on extra responsibilities, do relevant training etc.
Include a summary paragraph showing how you fit the person specification and are perfect for the job! x