The images here are all comic exaggeration. The millstone is described as "for an ass," which makes it the huge millstones that were dragged by an ass attached to them, going around in a circle. We cannot know if the "for an ass" had the same pejorative sense then as today, but clearly, the image is overkill and lost in translation. The word translated as "were better" and "be better" means "getting it together."
Jesus repeats the beginning of the two previous verses, but he changes the word used for "child" to "little ones" This is a play against the adjective "great" and verb meaning "lower" of the previous verse.
This verse is also interesting because of the differences among this version and Mark 9:42 and Luke 17:2. It holds a lot of clues about the source of Jesus's quotes in the Gospels (See Mark 9:42) and even how Jesus's lines changed over time. as he used them (See Luke 17:2)
Christ's Words in Matthew as a Guide to 40 Days of Prayer.