Socialization is critical.
A kitchen or family room is the ideal spot for a play pen. Exposure to daily noise and activity is half the job done, and ensures that youngsters are available to be played with whenever there's a spare moment. I don't do any leash work with very young puppies. They learn to follow my voice and movements while affording me a good opportunity to spot timid or independent individuals. Trotters tend to be better movers as adults.