A baby in a playpen
Puppies in a playpen on Enterprise-D
A playpen was an enclosure where young children could amuse themselves safely.
In 2270, Karla Five's father was kept in a playpen, as he was developmentally a baby by this point in his life. (TAS: "The Counter-Clock Incident")
In 2365, USS Enterprise-D's nursery had a playpen full of puppies. (TNG: "The Child")
In 2381, among the stress-relieving amenities on the Dove was a puppy playpen. (LD: "Room for Growth")