As a rule, dogs and children should always be supervised by a responsible adult. No dog should ever be left unattended with young children, and Beaucerons are no different.
While many Beaucerons are very tolerant and gentle with children, the simple fact is that Beaucerons are large, powerful dogs and even a young pup can easily knock a child down and injure them.
As with any dog, early and ongoing socialization is vital to establishing a positive association with children. Be sure to ask the breeder what their particular dogs are like around children or, better yet, observe their dogs interacting with children or ask for a videot showing their dogs interacting with children.
Keep in mind that some dogs may not be as tolerant with unfamiliar children as they would with children they know and trust.