The simple answer to that is yes, they can. Basically, if they are not happy with you for whatever reason, or if the numbers have changed and they have fewer kids, they can send you home.
The minimum hours they have to pay is four hours. If you are not happy to go home early, but you have not done anything wrong you can ask the nursery to let you stay the full day. It happens very rarely that they need to send you home because the numbers have changed, its more often they are not happy with the childcarer's work performance.
Please be always proactive, friendly, interested and as helpful as possible, because if they feel like there's no help whatsoever coming from you, they will ask you to leave and they won't pay you for the hours they booked you.