Parents are often concerned about the people their children will meet, and nurseries are well aware of this fact. They do background checks on their staff, and they work hard to ensure there is enough staff to adequately supervise the children. These are reasonable for this situation, but parents will continue to worry when they are not with their children. One way nurseries help ease the fears of parents is to ensure their children will be able to socialize safely.
While children are learning, they tend to be sitting at a table or desk in a classroom situation. This is a very controlled environment, so there is less risk a child will be subject to physical danger. Monitoring is constant in the room, so children are not as likely to be bullied. On the playground, it is more likely for small children to be outside the general society. This is where a good nursery will provide extra staff to ensure the situation stays friendly and sociable for all children.