I am not 100% sure about this but unless they are working less than 16 hours a week then they will be entitled to paid holiday even if they don't have a contract as the fact that the company is paying them means that the contract is implied. The less than 16 hours a week may have changed recently as well; it may be that everyone is now covered.
That they will almost certainly be covered by UK employment law doesn't mean that someone isn't trying to take advantage of them and if they are ignorant of the law, as maybe many overseas employees may be (I am thinking Polish and I speak as someone with Polish heritage), then that's not completely unlikely. You should definitely ask the nursery some searching and very challenging questions as again, the CRB issue would be top of my list.