A millstone was a heavy stone used for grinding of grain. It was so heavy a man couldn’t pick it up, a donkey would have to pull it. So, imagine the depth of Jesus’s feelings when He said that it would be better for someone to be cast into the sea with a millstone around their neck than to cause a little one to stumble. If you want to get Christ riled up, then mistreat or mislead a child.
It’s also interesting that Jesus not only talks about little ones as children, but also those little ones who believe. In other words, He is speaking of any adult that would take advantage of a child by misusing the child’s belief in God. (Example? Pope Benedict used this verse to refer to the Catholic Church’s scandal involving priests molesting children.)
But also note that “little ones” is more properly translated as “humble ones.” Now, that could be an adult that is mentally disabled, or limited. It could be that Jesus was even referring to new believers who are overjoyed about coming to salvation in Christ and then misled by a teacher who guides them astray by misinterpreting God’s word. Whomever it is that Jesus is referring to, He is quite clear. For these evil people, an agonizing death is preferable to leading the innocent into sin.