What is the term for a long, robe-like sleeping garment for infants?

The term for a long, robe-like sleeping garment for infants is best described as a "sleep sack." This type of garment is designed to provide warmth and comfort to infants while keeping them safe and secure during sleep. Sleep sacks often have a shape resembling a bag that allows for freedom of movement while ensuring that the child does not accidentally become tangled in loose blankets.

In contrast, a onesie is a one-piece garment that typically covers the torso and has legs, which is essential for daytime wear but does not fit the description of a long, robe-like sleeping garment. A kimono style may refer to a traditional Japanese robe but is not specific to infant sleepwear and does not offer the same protective purpose. A swaddle is a method of wrapping a baby snugly in a blanket to promote security and comfort, but it is not a garment in itself, nor does it have the robe-like qualities described in the question.

Overall, the sleep sack provides infants with a safe and snug sleeping environment, which is crucial for their sleep safety and comfort, making it the correct choice.