When you finished your w/exp at the nursery your supervisor would have completed a report on you - either in your w/exp diary or on a separate report sheet. If you did well then this report would be a good start to your CV- but to answer your question directly, it would be perfectly in order for you to ask the nursery for some sort of reference, even though you were not exactly 'employed' by them ,and providing your involvement with them was within the last six months or so.