This passage has been popping up around the internet among vigilante groups and alleged victim's rights groups, which they claim suggests that child abuse is worse than any other sin in God's eyes. My answe begins with the following warning:
Knowing this first, that no prophecy of the Scripture is of any private interpretation. -- 2 Peter 1:20
If anyone actually read Matthew 18, and the context it is used, you would see the meaning behind the passage is not what victim's rights groups are proclaiming:
The Greatest in the Kingdom of Heaven1 At that time the disciples came to Jesus and asked, “Who, then, is the greatest in the kingdom of heaven?”
2 He called a little child to him, and placed the child among them. 3 And he said: “Truly I tell you, unless you change and become like little children, you will never enter the kingdom of heaven. 4 Therefore, whoever takes the lowly position of this child is the greatest in the kingdom of heaven. 5 And whoever welcomes one such child in my name welcomes me.
Causing to Stumble6 “If anyone causes one of these little ones—those who believe in me—to stumble, it would be better for them to have a large millstone hung around their neck and to be drowned in the depths of the sea. 7 Woe to the world because of the things that cause people to stumble! Such things must come, but woe to the person through whom they come! 8 If your hand or your foot causes you to stumble, cut it off and throw it away. It is better for you to enter life maimed or crippled than to have two hands or two feet and be thrown into eternal fire. 9 And if your eye causes you to stumble, gouge it out and throw it away. It is better for you to enter life with one eye than to have two eyes and be thrown into the fire of hell. -- NIV
Jesus states two things. First off, in verses 3-5 Jesus compares the ideal Believer to a little child. In versus 6-9, Jesus is warning us about scandals in general. The Greek word σκανδαλίσῃ, translated as "stumble" in the NIV, is pronounced skandalisē. It is where we get the English term "scandal." Though we may immediately think of the Catholic Abuse scandal or maybe a political sex scandal, scandal is a general term for a large variety of activities. If you each your child to become a thief, it is a scandal. Jesus was warning again leading others into scandals by their actions.
What about the rest of the chapter? In Verses 15-19, we are taught the Christian method of dealing with Believers sinning in the Church. There are levels of rebuke in which multiple chances at forgiveness is possible-- one on one, then in the presence of a couple of witnesses, then to the church, and if all else fails separate yourself from the offender. The key here is forgiveness. It is a concept lost to modern America.
The last part of the chapter is the Parable of the unmerciful servant, warning us against squandering God's mercy. We have forgotten the concepts of forgiveness and mercy, though it is a very common theme in the Bible. After all, in Matthew 18:21-22 Jesus says not to forgive seven times, but seventy times seven (or seventy seven times depending on the translation). It does seem ironic how that message reminding us to forgive is in the exact same chapter as the verse abused by groups using the passage to promote agendas.
"If he sins against you seven times in a day, and seven times comes back to you and says, 'I repent,' forgive him." --- Luke 17:4 NIV
Scripture is highly symbolic. In Biblical times, numbers were symbolic, and the number seven means complete. What does it mean to forgive? THIS SITE can explain that far better than I can. I do know forgiveness is a process but it is also releasing yourself of the prison you make for yourself by perpetuating hatred. It is contrary to our vindictive attitude. You merely have to look at vendettas or blood feuds to see what happens when our desire for retribution for harms done goes unchecked.
Forgiveness does not mean we necessarily forget, but it does mean we let go of our need for vengeance, especially vengeance disguised as "justice."