Stumbling is caused by people tripping over something they do not expect ahead of them. Jesus had been talking about entering the kingdom of Heaven, and had used a little child as an example of simple trust and confident dependence. Then He gave four serious warnings about putting obstacles in people's way so that they would stumble and not enter God's kingdom.
 
The first warning was to anyone (religious leaders included) who would prevent children and adults with child-like faith from coming to Christ's kingdom, and continuing to grow in faith. The second warning was to worldly people, those who have not believed but prevent others from moving forward in a holy relationship with God, according to His Word.
 
Jesus' metaphor of the large millstone is very obvious. He was saying that the problem was so grave that the most radical action would need to be taken. Although their society disregarded children, and religious people disregarded people who followed Jesus, the Lord said that their attitude offended God greatly. His punishment would be far more severe.
 
In God's mercy, not all stumbling leads to falling. But Jesus wanted the disciples to understand that anything which obstructs the pathway of faith is such a serious matter that it needs radical action. It is still true today. The relationship between God and one of His 'little ones' (all of His children) is so precious that He will deal severely with those who seek to disrupt it. The warning is designed to ensure that the gospel is not restricted to the religious, wealthy or influential but is released without obstruction to the 'little ones' – all those with no power in this world, but who are precious to God.